The purpose of this study is to investigate the differences in the development level of physics concepts by students' proficiency level. The National Assessment of Educational Achievement (NAEA) of Korea, administered to the 6th and 9th grade students, from 2008 to 2012, were used for analysis. The test items showing significant differences by proficiency levels were extracted and analyzed in two ways. First, the distribution of responses to each distracter of the test items was compared by proficiency levels to identify misconceptions, preconceptions, or the tendency of thinking that students may have. Second, the percentage of correct choice of the test items, particularly developed for the same achievement standard, was compared to identify the degree of achievement by proficiency levels. The findings of this study revealed the specific contents that showed differences in the development of physics concepts by proficiency levels. For example, when comparing the "quickness" of objects whose distance and time are all different, the results showed that the lower level students of the 6th grade tended to consider distance to be an important factor, rather than time.