This independent study aimed to study service marketing mix affecting Thai Tourists in selecting spa service in Mueang Chiang Mai district. Population of this study was identified to 300 Thai tourists traveling to and taking spa services in Mueang Chiang Mai district. Questionnaires were used to collect data. Data analysis was conducted by the use of descriptive statistics, consisting of frequency, percentage and mean. The findings showed that most respondents were female. 32.3% of them were 41-50 years old; while 31.7% of them were 31-40 years old. They earned monthly income at the average amount of 30,001-50,000 Baht. The majority had travelled to Chiang Mai for 3-4 times; but did not always take the spa services. Purposes of taking such a service were to get relax and to be relieved from tension and stress. Type of spa service that they usually took was Thai massage. In each time, expense that they spent for such a service was less than 1,000 baht per a person. Most of them did not stay at a place where the spa service was served. For those who stayed at the hotel and spa type of accommodation, a reason of a group who took in-house spa services was referred to the better image of the in-house spa comparing to other standalone spa houses. In contrast, the reason of those who stay at the hotel and spa type of accommodation but took the services from outside was the interesting promotion and the professional services as served by spa therapists at those off-hotel spa houses. Source of information where they learned about the spa services that they took was from the Internet, including websites and social media, for example. The results of the study on service marketing mix factors affecting the selection of spa services indicated that the respondents ranked all following factors at high level of influence: physical evidence and presentation, process, people, product and price, place and promotion, respectively. Hereafter were shown the top-ten sub-factors affecting the Thai tourists to select the spa services: safety in every service process, privacy of spa service room, cleanliness of spa service house, music sound to enhance the relaxing atmosphere, good atmosphere of the spa house, cleanliness of spa clothing materials such as towel, bed sheet, gown and etc., to have professional spa therapists, to be certified for being the spa service provider with all standards by the Ministry of Public Health, to have spa therapists who had well knowledge of special cautions and restrictions for the health massage, safety of spa tools and devices which were cleaned well after used, appropriate time for waiting in each service step and convenient and rapid service process.