The purpose of this study is to examine the effectiveness of utilizing graphic organizers to enhance Thai EFL students' reading comprehension abilities and their opinions regarding using graphic organizers for reading comprehension. To gather information from sixty-four upper secondary school students at a public school in Bangkok, this research study used 1) instructional instruments: ten reading passages from a coursebook and ten graphic organizers designed by the researcher; and 2) research instruments: two English reading comprehension tests with multiple-choice, true/false, and short-answer questions, and focus-group interviews about students' opinions toward English reading instruction using graphic organizers. The data were analyzed using descriptive statistics, including mean scores, normalized gains, standard deviations, and inferential statistics, namely the dependent t-test and Cohen's d. The results indicated that the mean score on the reading comprehension post-test was significantly higher than the mean score on the pre-test at the 0.05 level of significance, and the experimental group acquired a higher score (M=87.00, SD=10.55) than the control group (M=51.18, SD=12.127) with the effect size of 0.846. Students exhibited a favorable opinion toward English reading instruction employing graphic organizers; and the average normalized gain was in the high gain range, g> = 0.78. Additionally, this research established that English reading instruction employing graphic organizers benefited students with low reading abilities and improved their abilities to comprehend what they read. The findings of this study provide potential avenues for improving students' reading comprehension.