Objective: Enhancing one's well-being is an important issue in mental healthpromotion. Many studies have found the efficacy of the positive intervention on the well-being or mental health in the healthy population. In this study, the efficacy of writing the happy diary was tested on a sample of college students. Method: We invited 19 male and 19 female college students to write the happy diary for 2 months during October 2014 to January 2015, which involved writing about one positive event every day. In addition, we used the life satisfaction scale (LS) and the brief psychiatric symptom rating scale (BSRS-5) as a mental health index at pre-test, at one month since writing the happy diary commenced, and at the end of two months. Result: Before writing the happy diary, the mean ± standard deviation of life satisfaction and distress scores were 18.6 ± 3.5 and 6.8 ± 3.7, respectively. After 1-month happy diary intervention, the mean ± SD of life satisfaction score was increased to 21.0 ± 3.6 and the distress score reduced to 5.4 ± 3.2, and a paired t-test revealed significant differences (T = 3.6, p = 0.001; T = -2.5 p = 0.05). After the 2-month happy diary intervention, the mean ± SD of life satisfaction score was increased to 22.4 ± 3.5 and the distress score reduced to 5.0 ± 3.6), and a paired t-test revealed significant differences (t = 7.6, p ＜ 0.001 and t = -3.5, p ＜ 0.001, respectively). Conclusion: Writing the happy diary was a simple and positive activity that could reduce mental distress and enhance life satisfaction. This finding could be useful to develop the programs of mental health promotion.