This research is based on Technology Acceptance Model (TAM), added to subjective norms and user satisfaction to examine the performance of information systems implemented in the government departments. It used questionnaire to collect users' opinions about the National Taxation Information System implemented in Taipei National Tax Administration, Ministry of Finance. The research findings show that the most users agree with the ease of use and usefulness of the system, and are satisfied with the system. The user's perceived ease of use has direct impact on the intention to use, also is partially moderated by perceived usefulness. The user's intention to use is also affected by perceived management participation of the sysem directly, and by the precived management involvement indirectly. In other words, both self-awareness and subjective norms influence user's intention to use, then affects user satisfaction. The study concludes that the perceived ease of use is the most significant impact factor on the intention to use and the user satisfaction.