Concept mapping helps learners to understand complex ideas. However, studies confirming that statement are scarce. A quasi-experimental and Design-based approaches were applied to understand the effectiveness of concept mapping based teaching method on grade eight students' conceptual understanding of photosynthesis. Two sections of grade eight consisting of 103 students were selected randomly. Pretest results were used to assign students as experimental and control groups and treated with concept mapping and lecture methods, respectively. The post-test results of the two study groups revealed a statistically significant difference indicated that students' treated with concept mapping method exhibited better conceptual understanding on the concept of photosynthesis than those treated with lecture method. This study suggests that science teachers should apply concept mapping method to teach science concepts systematically. The use of concept mapping method in science classes can help students to better understand the important concepts.