There are conflicting reports not only on the effect of scientific inquiry on academic achievement but also insufficient evidence on the relationship between understanding the nature of scientific inquiry and academic achievement; more generalizable studies are needed. The purpose of this large sampled study was to investigate the middle school graduates' understandings of scientific inquiry (SI) and the relationship between students' understandings of SI and academic achievement. The sample of the study (X = 3067) was selected by purposeful and layered-sampling techniques among ninth grade students studying at high schools with different orientations in Istanbul. Descriptive and inferential statistical approaches were used in the data analysis procedure. The students' academic achievement was operationally defined as the scores they received in the High School Entrance Exam (HEE). Views about scientific inquiry scale was applied to determine the participants' understandings about scientific inquiry. The data were collected in the first 2 weeks of the first semester of high school in accordance with the purpose of the study. The results of the study revealed that middle school graduates generally do not have adequate understandings of SI. Furthermore, according to the findings except for the "conclusion data conformity" aspect, there are statistically significant correlations between students' academic achievement and understandings in all aspects of SI.