Background: Previous studies have indicated that encouraging learners to teach to their peers has been proven effective in second language learning. However, to date, no research has focused on how learners teaching to peers of varying proficiency levels impacts video learning performance and the cognitive neuroscience mechanisms of learners. Objectives: The study investigated the effect of social comparison (upward comparison vs. parallel comparison vs. downward comparison) in video learning using learning by teaching. Methods: The study employed a within-subjects design, wherein participants taught English vocabulary to peers of three different ability levels in a second language setting. Students (N = 36 undergraduate and graduate students) were assigned to watch three videos in balanced order, then generate and speak out sentences to the peer. Results and Conclusions: The results of Repeated measures ANOVAs showed that learners in the upward and parallel comparison conditions showed higher learning performance and reported higher pressure than in the downward comparison condition. The results of EEGs showed that compared with downward comparison, upward and parallel comparison not only reduced the learners' theta oscillations in the left temporoparietal, occipital, or front-central but also manifested in the lower ratio of theta/alpha at the left temporal-parietal region, suggesting learners reduced mental workload. The findings have practical implications: teaching to peers with different levels can cause social comparison, and in video learning, learners should be encouraged to teach peers with higher-level or the same-level rather than the lower-level in order to improve their learning performance.