<b>Catalyzing the Scientific Content Mastery and Conceptual Understanding of Grade 11 STEM Students through Filipino Translated Comics in Earth Science</b> This action research aimed to determine whether a science comic written in Filipino is effective in stimulating scientific mastery of content and conceptual understanding of Grade 11 STEM students A.Y. 2017 2018. This research study made used of both quantitative and qualitative. For quantitative, a randomized pretest posttest control group design was used. Students were randomly selected and assigned to the control and experimental groups. The control group used the comics written in English while the experimental group used comics on the same topic written in Filipino. For qualitative analysis, an interview and journal narrative was used to gather the experiences of the participants of the study about the developed Filipino translated comics. The results revealed that students exposed to comics written in Filipino performed better than those who used the comics in English. They achieved a higher level of mastery of the contents. In terms of conceptual understanding, exposure to the comics in Filipino resulted to improved performance in the test for conceptual understanding. Pretest and posttest scores are positively correlated and mastery of contents is significantly correlated with conceptual understanding. Based on the result of the study, the following are recommended first, a large sample size should be used next time to increase the generalizability of the results. Second, only simple random sampling should be used and that samples should be randomly assigned to treatment groups. Finally, future research should investigate how comics written in Filipino promote conceptual understanding. Conceptual Understanding, Content Mastery, Earth Science, Filipino Translated Comics 42-44 Issue-2 Volume-4 Ian Ismael E. Marces