The study titled “Causal factors related to underground lottery purchasing behaviors of Chompoo Samakki Credit Union Cooperatives members” aimed to explore underground lottery purchasing behaviors of members of Chompoo Samakki Credit Union Cooperatives, and to investigate causal factors related to illegal lottery purchasing behaviors of members of Chompoo Samakki Credit Union Cooperatives. The study was conducted on the basis of qualitative research using document analysis technique, non-participant observation, an in-depth interview, and focus group discussion. The findings from the study showed that underground lottery purchasing behaviors of Chompoo Samakki Credit Union Cooperatives members could be divided into 13 points as follow: (1) Most of the key informants purchased both government and illegal lotteries of every draw, (2) Most of the key informants purchased underground lotteries almost every draw (3) Most of the key informants purchased illegal lotteries within a budget of 200-500 baht per draw, (4) Most of the key informants purchased underground lotteries from underground lottery hosts but some of them purchased illegal lotteries from online illegal lottery gambling, (5) payment methods were pay all promptly , pay after the draw or pay after two draws. However, purchasing underground lotteries from online underground lottery gambling, they were required to pay all promptly, (6) two types of choosing underground lottery hosts were (a) underground lottery hosts who are ordinary persons, such as friends, relatives or familiar persons, and (b) underground lottery hosts that are Applications or websites, (7) 3 models for selecting the numbers for underground lotteries were home address numbers, numbers displayed on vehicle number plates, and ages of family members, (8) Most of the key informants purchased all forms of underground lotteries in terms of “the first and last digit numbers” or “2-digit numbers up and 2-digit numbers down” and both “alternate numbers” and “exact numbers”, (9) Most of the key informants set a budget for purchasing underground lotteries at the maximum of 1,200-1,500 baht per draw; 200-500 baht per person per draw on an average, (10) Most of the key informants won the underground lottery prize very often; every other month or every other draw, but some of them once in a while won the underground lottery prize; 2-3 times per year, (11) Most of the key informants won the underground lottery prize per draw at the maximum of 5,000-6,000 baht per draw, (12) Most of the key informants were superstitious when purchasing underground lotteries, especially the belief in winning the underground lottery prize of a certain draw, (13) Most of the key informants felt indifferent towards losing the underground lottery prize as they thought they are able to win the lottery prize for the next draw. There are 3 significant factors affecting underground lottery purchasing behaviors of Chompoo Samakki Credit Union Cooperatives members, namely, (1) factor related to underground lottery itself, (2) internal factor of underground lotteries, and (3) external factor of underground lotteries. Factor related to underground lottery itself is divided into 4 minor factors as (a) easiness of choosing numbers, (b) easiness of purchasing underground lotteries, (c) they spend less money purchasing underground lotteries but receive prize money equal to or greater than purchasing government lotteries and (d) underground lotteries offer a discount or commission to buyers. Internal factor of those who purchase underground lotteries is divided into 3 minor factors, i.e. (a) knowledge about laws on gambling, (b) underground lotteries are enjoyment rather than striving for income and (c) purchasing underground lotteries is taking a chance rather than gambling. The last factor is external factor of those who purchase underground lotteries, divided into 3 minor factors as (a) influence from friends, (b) influence from media, and (c) influence of gambling they used to play in the past.