The purpose of this study aimed to construct an online competitive game-based learning system by using freeware for junior high school students and to assess its effectiveness. From the learning standpoints, game mechanisms including learning points, competition mechanism, training room mechanism, questioning & answering mechanism, tips, and feedback mechanism are taken into consideration while constructing the system. The system contains screens of Log-in, Game lobby, Waiting room, Player's room, Question & Answer, and Scoring. After the system was established, it was implemented in a 10 week teaching experiment. A total of 35 junior high school students participated in this teaching experiment. Both pre-test and post-test were administered and analyzed. A 5-point Likert scale questionnaire, containing domains of system operation, learning effectiveness, competition and incentive, and training room learning was also included to assess user's satisfaction. Descriptive analysis and independent t test were used to analyze the collected data. The findings of the study show that most students are satisfied with the four domains of the freeware constructed online competitive game-based learning system. Meanwhile, the online competitive game-based learning system is effective for junior high school students' learning. (Contains 5 tables and 9 figures.)