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The purpose of this study was to identify the relative predictive influences of prior knowledge, meaningful learning orientation, formal reasoning ability and mode of instruction on understanding in photosynthesis and respiration in plants concepts. Two hundred thirty three 8th grade students from six classes of one elementary school in Ankara participated in this study. The study was carried out during the 2004-2005 Fall semester. Students in the experimental group (N=117) received conceptual change instruction, and the students in the control group (N=116) received traditional instruction. Two-tier multiple choice diagnostic test, &ldquo / Photosynthesis and Respiration in Plants Concept Test&rdquo / developed by Haslam and Treagust (1987), was used to determine students&rsquo / understanding of photosynthesis and respiration in plants concepts. The test was administered to the sample prior to the treatment as pre-test, and after the treatment as post-test. The pre-test scores were used as prior knowledge of students. Students&rsquo / reasoning abilities were measured by the &ldquo / Test of Logical Thinking&rdquo / and their learning orientations were measured by &ldquo / Learning Approach Questionnaire&rdquo / . The results of the study indicated that students held several misconceptions concerning photosynthesis and respiration in plants concepts. Significant differences between the experimental group and control group with respect to understanding of the concept were found in favor of experimental group. The main predictor of achievement in the experimental group was students&rsquo / prior knowledge, while it was reasoning ability in the control group. Meaningful learning orientation accounted for a small amount of variance in the experimental group but it did not contribute to understanding on post-test scores in traditional group.