This study aimed at investigation of the development of elementary science teachers' subject matter knowledge through some selected hands-on activities based on inquiry-based instruction. 58 voluntary elementary science teachers participated in the study with non-random convenience sampling. The elementary science teachers in the study carried out 15 inquiry-based hands-on physics activities. To assess Subject-Matter Knowledge (SMK), three instruments about some selected physics content (mechanics, optics, pressure, electricity, and heat) and science process skills were utilized as pretests and posttests: Traditional Problems Test (TPT), Context-Based Problems Test (CBPT), and Science Process Skills Test (SPST). Scores, particularly on the TPT and CBPT, indicated unsatisfactory levels of SMK in physics. Furthermore, performances were better on the TPT rather than the CBPT. In consequence, the inquiry-based hands-on physics activities were observed to help the elementary science teachers improve their SMK especially in terms of the CBPT scores. Finally, the SPST scores failed to reflect the progress in science process skills probably due to fatigue of testing. In sum, engagement in inquiry-based hands on physics activities seems to help the elementary science teachers develop especially their substantive knowledge.