The purpose of this study is to examine grade differences in the development of postformal thinking and test if there is interaction between grade and sex on developmental trend exists. Subjects were 402 college students including freshman, sophomore, junior and senior. And the research instrument was the Social World Beliefs Inventory that measures the cognitive levels of formal thinking and postformal thinking. Logit loglinear model was conducted as a method for data analysis. The results displayed the grade’s main effect model was the most suitable for explained the empirical data. Yet the sex’s main effect model, the sex and grades’ main effect model, and the interaction model between sex and grade were not the appropriate models to explain the empirical data. In short, the proportion of postformal thinking group relative to formalistic thinking group was increased from freshmen to seniors and the grade’s developmental differences of postformal thinking didn’t interact with sex. The possible reasons of above findings are discussed in the paper. According to these findings, there are implications and suggestions for future research mentioned.