The purpose of this study is to develop a two-tier multiple choice test to detect tenth grade high school students' conceptual understanding related to chemical equilibrium. In the development of the test the steps described by Treagust (1988) were used. The test was prepared for the purpose of detecting students' conceptual understanding in topics like approach to equilibrium, equilibrium state, changes in the equilibrium conditions, addition of inert gas and catalyst. The test was administered to 102 tenth grade students. The internal validity coefficient of Cronbach alpha was 0.903. Difficulty indices ranged from 0.20 to 0.93, and discrimination indices ranged from 0.19 to 0.89. Each item was analyzed to determine student understanding and identify alternative conceptions about, chemical equilibrium concept. Results of the test suggested students did not acquire a satisfactory understanding of chemical equilibrium concepts and 25 common students' alternative conceptions were identified through analysis of test items.