Background and purpose of the study: STEM learning often involves a multitude of complex and abstract concepts and ideas that can be challenging for students to comprehend. Research suggests that the oral and visual representations in video lectures can maximize students' cognitive infrastructure, helping them to organize knowledge more effectively. However, compared to traditional learning methods, video lectures may lack interaction and feedback, which can lead to ineffective learning strategies (e.g., passive viewing) and reduced learning engagement. Instructional explanations serve as a generative strategy, enabling students to create oral and written pieces based on the knowledge gained from video lectures and their prior knowledge. This study recruited a total of 87 undergraduate students and explored how the modality of instructional explanations generated by these students for a fictious student influenced their learning. Specifically, the study explored the effects on students' learning performance, attention, behavioral patterns of preparing-to-explain, the quality of notes, and the quality of instructional explanations in video lectures on a STEM subject. Results: The results revealed that students who adopted a combination of oral and written instructional explanations showed better immediate retention and transfer than those adopted just one type of explanation. In addition, both oral-only and combined oral-and-written explanations promoted more self-regulated learning behaviors during the phase of preparing-to-explain. The study also found that the quality of instructional explanations played a mediating role in the effects of modality. Conclusions and potential implications: Our findings suggest that combining oral and written instructional explanations is more effective in supporting students' STEM learning from video lectures compared to using a single form of explanation. These findings have significant implications for teaching and learning STEM subjects through video lectures. Students and educators should recognize the complementary roles of oral and written instructional explanations and opt for a combined oral-and-written approach during STEM learning activities.