Objectives : The purpose of this study is to develop a job performance assessment tool for dental hygienists. Methods : The development consisted of three steps. The first step was to construct the contents of the assessment tool and a pretest questionnaire. In the second step, validity and reliability tests were done and modified for the pretest questionnaires and an assessment tool was made. The third step was verification of the assessment tool. A total of 678 data from the dental hygienists were analyzed to verify the validity and reliability. The assessment tool consisted of two areas such as job performance and competency/attitude: the job performance area covered four sub-areas including oral health education, preventive dental care, assistance for dental treatment, and administrative management. Results : The number of questions and reliability test result for each sub-area were as follows. Reliability coefficient for oral health education (10 questions) was 0.899, preventive dental care (8 questions) was 0.861, assistance for dental treatment (18 questions) was 0.915, administrative management (8 questions) was 0.919, competency of dental hygienists (11 questions) was 0.947, and attitude of dental hygienists (15 questions) was 0.955. Cronbach ${alpha}$ coefficient for reliability of total 70 questions of the assessment tool was 0.980. Conclusions : The validity of the assessment tool was verified by three steps and the reliability proved to be significant. Consequently the assessment tool was found to be useful as an objective and valid job performance assessment tool for the dental hygienists.