This study explores the effect of user and system characteristics on our proposed web-based classroom response system (CRS) by a longitudinal design. The results of research are expected to understand the important factors of user and system characteristics in the web-based CRS. The proposed system can supply interactive teaching contents, quizzes, and questions from its database. In addition, the system provides three playful classroom response modes--the individual assigned mode, the rush answering mode, and the pushing button mode--to increase computer playfulness, friendly interface, and interactivity for students. Both user and system characteristics are based on the external variables from the technology acceptance model (TAM). The results of the longitudinal empirical study indicate that the effect of user characteristic on perceived usefulness is more than that of system characteristic, while the effect of system characteristic on perceived ease of use is more than that of user characteristic. Furthermore, the difference between short-term use and long-term use is analyzed. In the user characteristic, the effect of long-term use on perceived usefulness is more significant than that of short-term use. The effect of system characteristic on perceived usefulness is insignificant for long-term use, but that is significant for short-term use. The effect of long-term use on perceived ease of use is more significant than that of short-term use. In addition, computer playfulness is positively related to perceived ease of use, while subjective norm affects directly on perceived usefulness. (Contains 4 tables and 6 figures.)