From a constructivist point of view, the importance of MUDs (Multiple User Dungeons) in education is justified based on their community-forming, learning, and role-playing functions. The authors propose a typology for educational MUDs and discuss their individual instructional approaches in order to measure MUD potential in ten-os of learning-specific angle. According to the metaphor, technology, and instruction we found in educational MUDs, we categorize them as teacher-centered, learner-centered, environment-centered, or construction-centered. After describing the learning structures and characteristic features of our classification system, we use three MUDs developed and tested in Taiwan to demonstrate the advantages and disadvantages of using a MUD-based learning approach. (Contains 2 charts and 2 diagrams.)