This paper presents the effect of simulation-based inquiry (SimIn) integrated with formative assessment (FA) on students' conceptual learning of buoyancy-driven phenomena. In this study, we examined students' conceptual learning performance from two Thai public schools collected in two studies. In order to broaden the scope of the study's method, a two-step experiment was conducted: the first study recruited 120 ninth-grade students who agreed to participate, and they were assigned into two experimental groups and one control group. The control group received only SimIn, without integration of FA; the first experimental group participated with SimIn integrated with FA method of agree and disagree statements (SimIn-FA A&D ); and the second experimental group participated with SimIn integrated with FA method of agreement circle (SimIn-FA AC ), involving group participation. A multiple-group pretest-posttest experimental design was used, and data were analyzed using one-way ANCOVA, plus a protocol analysis was done to verify the value of the FA integration. The second study compared 39 twelfth-grade students' conceptual understanding scores, and investigated the pattern of, and the quantitative changes in, the process of conceptual change based on a one-group, pretest-posttest method. The result showed that students' conceptual understanding scores were significantly improved after participating in the experimental learning. Moreover, the integration of FA into SimIn produced a better progression of scientific understanding than without FA. This finding suggests that the integration of FA into SimIn can be used to effectively & Niwat Srisawasdi