The study investigated the effect of an instructor's positive (vs. neutral) emotions in video lectures on student learning using either easy or difficult geography topics (i.e. easy: the Earth within the universe; difficult: understanding time and date calculations of Earth). The results showed that, first, students responded more positively towards the video lectures in which the instructor demonstrated a positive emotional state, compared to videos with a neutral instructor, regardless of content difficulty. Second, the content difficulty of the video lecture moderated the effect of the instructor's positive emotion on students' retention performance. Notably, for the easy content, the instructor's positive emotion significantly enhanced retention performance. Third, the instructor's positive emotion in the both easy and difficult video lectures improved students' transfer performance. This beneficial effect was found to be mediated by students' perceived attention during the learning process. These findings contribute to our understanding of the effects of emotional design in video lectures. Practical recommendations drawn from these findings should be interpreted with consideration of the study context and complemented with other empirical evidence, however one interpretation of these results can be that instructors should consider displaying positive emotions while teaching, particularly when delivering easier content, to improve both retention and transfer performance among students.