The prevailing view on educational stratification in recent decades is that inequality in educational opportunity has persisted despite educational expansion. This paper examined these trends in Japan after World War II and tested some stratification mechanisms by applying ordered logit models, especially "partial proportional odds models." The data from the 2005 Social Stratification and Mobility (SSM) survey were used. The major findings are as follows: (1)The effects of socioeconomic variables—father's occupation, high parental education, and family possessions—on educational attainment clearly decreased in the middle-aged cohort, which comprised individuals who had finished compulsory education after the high economic growth of the 1960s. (2)Although the effects of parental education partially increased in the younger cohort, which comprised individuals who entered high schools and universities after the mid-1980s, the effects of family possessions continued to decrease and the effects of father's occupation showed little change. From a long-term perspective, we may conclude that social inequality in educational opportunities has been decreasing. In addition to the trend analysis, we examined a "relative risk aversion" (RRA) hypothesis. To test the hypothesis, we specified a necessary condition—namely, that there must be an interaction between the socioeconomic variables and thresholds in the ordered logit models. (3)On these grounds, we found that the effect pattern of parental education consistent with the RRA hypothesis. However, this is not the case for father' s occupation. We concluded that the RRA hypothesis was irrelevant in Japanese educational stratification, insofar as it was speculated in the context of occupational mobility.