The purpose of this study is to examine public and private middle school students' levels of Health-Related Fitness Knowledge (HRFK) according to school type, gender, and grade. A cross-sectional survey method was applied in the research. A total of 334 public middle school students (n(female) =154 and n(male) = 180) and 386 private middle school students (n(female) = 187 and n(male) = 199) participated in the survey. The data collection instrument was developed by Hunuk and Ince (2010) based on the "Superkids-Superfit Knowledge" study (Mott, Virgilio, Warren and Berenson, 1991). The data collected was analysed using the following descriptive and non-parametric tests: the Pearson chi-square, Mann-Whitney U-test, and Kruskal-Wallis H-test. Findings indicated a significant difference according to school type and age group (p<.05), but a non-significant difference according to gender and HRFK test result. Results improved year to year except among 7th graders. In other words, private middle school students' HRFK results were higher than those of public middle school students; grade level was also linked to HRFK, but gender was not. These results suggest that physical education curriculums should be developed with reference to HRFK objectives. Another recommendation would be that HRFK tools be customized by grade level in the Turkish context.