This study sets out to assess the scientific literacy of secondary school students and to describe their differences according to gender, grade, course. This study involved 112 middle school students and 213 high school students. Their scientific literacy was measured by the Scientific Literacy Test designed by Manhart (1997). A 70-item multiple-choice test was used to assess their scientific literacy. The constructs of science factor included 36 items making up physical science, life science, and earth science subtests. The social aspects of science factor consisted of 34 items in nature of scientific inquiry/knowledge, science as a human endeavor, science and technology, and societal perspectives. A two-way analysis of variance (ANOVA) and t-test were conducted using the SPSS program. The scientific literacy score of the middle school students was 45.17. There was no significant difference according to gender but boys tended to perform better than girls on both the constructs of science factor and the social aspects of science factor. The scientific literacy score of the high school students was 51.79. There was no significant difference according to gender. But, boys tended to perform better than girls on the constructs of science factor. Girls tended to perform better than boys on the social aspects of science factor. The students taking a course on natural science got statistically higher scores than the students taking a course on humanities. The high school students got statistically higher scores than the middle school students.