The use of instant messaging to support e-learning will continue to gain importance because of its speed, effectiveness, and low cost. This study developed an MSN agent to mediate and facilitate students' learning in a Web-based course. The students' acceptance of the MSN agent and its effect on learning community identification and learning achievement were investigated using the technology acceptance model. Results indicated that the MSN agent proved easy to use, and that students recognized its benefits with regard to their learning. In more detail, students perceived that the MSN agent would be beneficial to learning community identification, but this perceived usefulness had less effect on their learning achievement. However, it was found that perceived system usage of the MSN agent was significantly related to learning achievement, which differed from the result of analyzing students' perceived usefulness of the agent. Rather than using a systematic recording log, the perceived system usage of the MSN agent was measured through questionnaires. Interview feedback revealed that effective learning was related to the learners' engagement in the MSN agent and not just from their perceived usefulness of it. Therefore, the MSN agent could have more potential to facilitate students' learning in an online environment if learning activities related to study are designed to promote students' engagement.