The primary purpose of this study was to assess understanding of concrete and formal operational science content by concrete and formal operational students in secondary school physical science classes. To carry out this study subjects were selected from junior high schools of over 2500 students, and they were identified as concrete, transitional, or formal operational using GALT(Group Assessment of Logical Thinking) developed by Roadrangka, Yeany and Padilla(1983). Instructional objectives were extracted from the science content taught during the second term of the academic year in the 7th, 8th and 9th grade, and they were classified as concrete or formal operational. Written test involving those objectives were constructed and administered. The results of this study suggested that formal operational thought brought deeper understanding to concrete science content as well as formal science content. Differential effects by sex on understanding of both concrete and formal science content were contradictory through the grade levels. Results of multiple comparison tests suggested that students categorized as formal operational demonstrated no better understanding of concrete science content than those categorized as transitional. However, they demonstrated better understanding of formal science content.