The purposes to conduct this research were to study the relationship between some particular factors and Critical Thinking and to express the description in the variance of correlation coefficients from research documents related to Critical Thinking. 57 samples of experimental and correlational research documents that published in B.E. 1982-2004 were synthesized. To synthesize those, 17 research characteristic variables were gathered by employing research characteristic recording form, and quality research evaluation form. Then the GLASS method was employed to synthesize all variables for correlation coefficients. There were 90 correlation coefficients; 36 correlation coefficients of Learning and Teaching Factor, 38 correlation coefficients of Learner Factor, 16 correlation coefficients of Individual and Caring Factor. Statistics employed in this study were descriptive statistics, analysis of variance (ANOVA) and stepwise multiple regression analysis. The results of research synthesis were: 1. The Factors indicating the highest related to critical thinking was the Learning and Teaching Factor (r[bar] = .366) consisted of 3 sub-factors; there were Teaching Materials ([r-bar] = .429), Teaching Method ([r-bar] = .391), Learning and Teaching Environment ([r-bar] = .287); followed by Learning Factor ([r-bar] = .334) consisted of 5 sub-factors; there were Problem Solving Ability ([r-bar] = .541), Nursing Ability ([r-bar] = .427), Reading Ability ([r-bar] = .423), Achievement ([r-bar] = .356), Cognitive Skills ([r-bar] = .243); The factors indicating the least related to Critical Thinking was Individual and Caring Factor ([r-bar] = .237) consisted of 3 sub-factors; Attitude and Belief and Behavior Factor ([r-bar] = .281), Caring Factor ([r-bar] = .265) Individual Factor ([r-bar] = -.093). There was a positive relationship between the three main factors and Critical Thinking ([r-bar] = .310). 2. The research characteristic variables which significantly accounted for the variation in the effect size were Division Producing Research, Course Taught, Learner Factor and Statistics Test. There were 6 research characteristic variables could simultaneously explain 51.5 percents of variation in the effect size. 3. The research finding indicated that Teaching Method and Teaching Materials were the factors which had more positively influence in Critical Thinking than other factors among Learners at primary, secondary, and undergraduate levels. Such Critical Thinking was also the factor which had more positively influence in Problem Solving Ability than other factors among Learners at primary and secondary levels. Last but not least, Attitude Belief and Behavior was only factor that negatively influence in Critical Thinking among the primary level.