This research applies a web-based multiplayer online game to motivate students doing self-assessment when they are studying. The experiment is conducted in the first and second year undergraduate programming language courses in Taiwan. Within the multiplayer online game, students can explore the game world, chat with classmates, and solve quests. The quests in the game are self-assessment items, however when the quest items are presented in game form, students feel they are not taking quiz but playing. We collect students' demographic information and their game-play log to evaluate the effectiveness of such multiplayer assessment game. The results show that gender and having internet at rental place do influence student's intention to play the game. Students who have better academic achievement (i.e. higher scores) also have much more willing to play the game during the semester.