Abstract：This study examined the effect of optimism-pessimism and affectivity on sport performance and burnout in college athletes, and the mediating role of positive and negative affectivity. The research goals were: (1) to explore the effect of optimism on positive affectivity and sport performance of college athletes and the mediating effect of positive affectivity; (2) to explore the effect of pessimism on negative affectivity and burnout of college athletes and the mediating effect of negative affectivity. A total of 302 colleges athletes' data in Taiwan were collected using survey techniques. The research instruments included 3 kinds of scales of optimism and pessimism, affectivity, and burnout with a self-report questionnaire of sport performance. The results showed that: (1) Optimism can effectively predict positive affectivity and sport performance. Positive affectivity can also effectively predict sport performance, with a full mediation effect. (2) Pessimism can effectively predict negative affectivity and burnout. Negative affectivity can also effectively predict burnout, with a partial mediation effect. The findings not only support positive psychological theory, but also indicate that the optimism and pessimism expectancies of athletes regarding the future will significantly influence satisfaction and burnout of sport performance through the mediators of positive and negative affectivity. (Full text)