Studies in science education aimed at determining students understanding of biology concepts have revealed many misconceptions. Misconceptions are resistance to change, well embedded in an individual's cognitive ecology, and difficult to extinguish even with instruction designed to address them. The present study concerned primarily with students' misconceptions, and with instructional strategies to affect the learning of scientific conceptions and to affect conceptual change from alternative to scientific conceptions. This study examined the contribution of conceptual change oriented instruction, previous learning in biology, and logical thinking skills to students' understanding of ecology concepts. The subjects included ninth grade students who were enrolled in four classes of the general biology course in a high school. Each of two instructional methods was assigned to two classes. The classes receiving the traditional instruction were considered as the control group. The classes utilizing the conceptual change instruction were called as the experimental group. The conceptual change oriented instruction involved conceptual change texts accompanied with demonstration in small group work. This instruction provided learning environments, such as identifying common misconceptions, activating students' misconceptions by presenting examples, presenting descriptive evidence that typical misconceptions were incorrect, and providing a correct explanation of situation. The Ecology Concept Test was developed to identify students' misconceptions and their understanding levels. This test was developed by investigating related literature and interviews with students. The Multiple Regression Correlation Analysis indicated that treatment, previous learning in biology and logical thinking skills each made a statistically significant contribution to students' understanding of ecology concept. The conceptual change oriented instruction caused a significantly better acquisition of scientific conceptions related to ecology concepts than the traditional instruction. Also, this study revealed some students' misconceptions before and after treatment. Key Words: Assessment, misconception, conceptual change approach, ecology.