The demand on quality management systems (QMSs) is brought by increasing client needs and complexity of construction projects. ISO 9000, a recognized quality management system often used in quality assurance, has been adopted by several construction companies to help ensure work quality. However, a suitable tool for assessing quality management practices, particularly, ISO 9000-based QMSs, in the construction industry is still limited. This research aims to contribute to the continuous search for the best possible way of assessing quality management practices. Towards this objective, this research has been designed into two phases. First is the development of tool and criteria for assessing ISO 9000-based quality management practices. The proposed tool in this research is adopted from the concept of effectiveness in previous studies. Effectiveness of ISO 9000-based QMSs is assessed based on achieving the specified QMS requirements derived from the ISO 9001:2000 standard and quality goals. Two different sets of scales to aid in the evaluation were also developed and are highlighted in this research. The development of tool and criteria is derived from literature review and interview with 14 professionals involved in ISO 9000. From data collection, thirty-three (33) quality principles, ten (10) prescribed company quality goals, and criteria for evaluation were developed. The second phase is the evaluation of quality management practices using the data collected from twenty-three (23) ISO 9000 certified contractors in Thailand. The result illustrates that there are different levels of practice among contractors using the proposed tool for evaluation. Results show that among the quality principles, contractors need to improve on client satisfaction. Meanwhile, good results were obtained in the control of monitoring and measuring devices. For the quality goals, the respondents need improvement on achieving better quality of work. The proposed tool in this research can help construction companies to identify their constraints and improve areas of their quality management practice.