Conceptual understanding is a gateway to acquire relevant knowledge and ability to apply this knowledge across a variety of circumstances. The main purpose of this quasi-experimental design based research was to understand the effectiveness of concept mapping based teaching method on grade eight students’ conceptual understanding of photosynthesis. Out of four sections, two sections of grade eight consisted of 103 (male =33 and female =70) students were selected randomly at Ewket Fana primary school, Bahir Dar town. Students were purposively assigned as experimental group (N = 53) and control group (N = 50) based on their pre-test results. The experimental and control groups were treated with CM and traditional (lecture) methods, respectively. Pre-post photosynthesis tests, classroom observation, informal assessment and concept mapping activities were used to collect data. The data were analyzed quantitatively through descriptive and inferential statistics (t-tests), and qualitatively through narration. The pretest scores of the two groups were not statistically significant (p = 0.60); this signifies that the two groups had equivalent ability on the concept of photosynthesis before treatments. But the post-test results of the two study groups revealed a statistically significant difference (p= 0.00) that suggests students’ treated with CM method exhibited better conceptual understanding on the concept of photosynthesis than those treated with traditional method. Similarly, in post informal classroom assessment students’ in experimental group could give better explanation and reason for the open-ended questions than control group students. From the results, it is possible to conclude that CM method was more effective than the lecture method towards students’ conceptual understanding in science in general and photosynthesis in particular. Therefore, this study suggested that science teachers should apply CM method to teach science concepts systematically.