Background: Video lectures which include the instructor's presence are becoming increasingly popular. Presenting a real human does, however, entail higher financial and time costs in making videos, and one innovative approach to reduce costs has been to generate a virtual speaking instructor. Objectives: The current study examined whether the use of a virtual instructor in video lectures would facilitate learning as well as a human instructor, and whether manipulating the virtual instructor's characteristics (i.e., voice and appearance) might optimize the effectiveness of the virtual instructor. Methods: Our study set four conditions. In the control condition, students watched a human instructor. In the experiment conditions, students watched one of (a) a virtual instructor which used the human instructor's voice and an AI image, (b) a virtual instructor which spoke in an AI voice with an AI image made to speak using text-to-speech and lip synthesis techniques, or (c) a virtual instructor with used an AI voice and an AI likable-image of an instructor. Results and Conclusions: The AI likable instructor condition had a significant positive effect on students' learning performance and motivation, without decreasing the attention students paid to the learning materials. Implications: Our findings suggest that instructional video designers can make use of AI voices and AI images of likable humans as instructors to motivate students and enhance their learning performance.