This study investigated the effectiveness of student-centered instruction on eighth-grade students' conceptual understanding of photosynthesis. The study employed a Quasi-experimental research method. The data collection instruments included were multiple-choice tests, observations, and an informal assessment. The pretest and posttest were administered before and after treatment, respectively. An informal assessment was also administered before and after the intervention. Besides, the observation was conducted from the first day up to the last day of the intervention. The pretest and posttest data were analyzed using descriptive statistics and t-tests quantitatively, while data collected through informal assessment and observation were presented using narration. The findings of the pretest revealed that there was no significant difference between the mean scores of the two groups. Post-test results revealed that experimental group students significantly improved their conceptual understanding of the topic of photosynthesis compared to the control group students. The study concluded that the guided inquiry teaching method is more effective than the lecture method in improving students' conceptual understanding of photosynthesis. Based on the findings of this study, it was recommended that governments encourage biology teachers to apply guided inquiry teaching methods by providing appropriate instructional material.