The main objective of this study was to investigate the effect of using guided inquiry teaching method in improving grade eight students’ conceptual understanding of photosynthesis at Dill Chibo primary school. The study employed Quasi-experimental research method. The population of the study consisted of 217 grade eight students in four sections. Out of which 101 students in two sections were randomly selected as a sample and assigned as 51 students for experimental group who were taught by guided inquiry method and 50 students for control group who were taught by the lecture method for three weeks. The data collection instruments were multiple choice tests, observation, as well as informal assessment. The pre-test and post-test were administered before and after treatment respectively. The informal assessment also administered before and after intervention. Besides, the observation was conducted from the first day up to the last day of the intervention. The pre-test and post-test data were analyzed using descriptive statistics and t-tests quantitatively while; data collected through informal assessment and observation were presented using narration. The finding of pre-test revealed that there was no significant difference between the mean score of the two groups. The post-test result revealed that experimental group students significantly improved their conceptual understanding of the topic photosynthesis compared to the control group students. The study concluded that guided inquiry teaching method is more effective than the lecture method in improving students’ conceptual understanding of photosynthesis. Based on the finding of this study, it was recommended that governments should encourage biology teachers to apply guided inquiry teaching method by providing appropriate instructional material.