The relationship lying between critical thinking and creative thinking is opposite or complementary, results of previous relevant researches have not yet concluded. However, most of researches put the effort to compare the respective effect of the thinking methods, either the teaching of creative thinking or that of critical thinking. Less of them showed the interest to investigate the combined effect of these two thinking skills teaching, especially its synergy. Therefore, present study aimed to discuss the synergy of critical thinking and creative thinking, and investigated the joined effect of these two methods of thinking in the courses of "Integrated Activity courses". Not only the separate influence but also the synergy would be our interests. Moreover, the outcome would also be compared with the learning result of single creative thinking skill teaching. Participants were 147 male students and 118 female students of ninth grade from 8 intact classes in a public middle school in Taiwan, the number of total participants were 265. Experimental design was "nonequivalent control group pretest/post-test quasi-experimental design". Participants were assigned into three groups, three intact classes for creative thinking teaching group (EG1), three intact classes for "critical thinking and creative thinking combined teaching group (EG2)" and two intact classes for regular teaching method group (watching video, control group). All participants took the assessment "The New Creative-Thinking Testing" and "The Critical-Thinking Testing, Level 1" after receiving the courses as post-test. Data were analysed statistically by one-way ANCOVA. The results showed that: (1) Regards to the performance in Integrated Activity courses, no significant difference was found among students in EG1 and those in EG2. However, the students in both experimental groups got higher scores than those in control group did. (2) As for the ability of critical thinking, students in EG2 got higher score than those in EG1 and CG did, but no difference was found among the students in EG1 and those in CG. (3) In respect of originality, students in EG2 got higher score than those in EG1 did, the influence of critical thinking was significant. Theoretical and possible applications of the results were also discussed. Present study not only manifests the correlation between creative thinking and critical thinking, but also provides the empirical data for synergy in actual pedagogical situations.