This study was conducted to develop a rating scale for assessing the teaching competency of elementary school preservice teachers at the Department of Education, National Hsinchu University of Education, Taiwan. In this study, the rating scale was designed using questionnaires that were developed through applying the Delphi method. Delphi data analysis was conducted by 17 experts over 3 rounds, after which adjustments were made to improve the rating scale. Subsequently, preservice teachers into the fourth year of their study were recruited to test the rating scale. Two tests were conducted, each involving 8 preservice teachers and 3 rating staff, and the scores for the 2 rating tests were collected for analysis. Interrater reliability was tested using the Kendall’s W harmonic coefficient test, and the preservice teachers and rating staff were interviewed to revise the rating scale indicators. The rating scale proposed in this study comprised 4 dimensions (i.e., instructional design, teaching methods and techniques, teaching media and communication, and classroom management and teacher-student interactions) and 20 indicators. A 5-level assessment rubric was formulated to rate the 20 indicators. Finally, method of using the rating scale was outlined in the paper for related personnel to use the scale correctly. The proposed rating scale can be used by teacher education institutions to assess the teaching competency of preservice teachers. In addition, the scale can be used as formative assessment instruments to assist preservice teachers in improving their teaching competency, or as an evaluation instrument for education institutions hiring teachers.