Psychological assessment is one of the core competencies in professional psychology and an important training domain for clinical psychologists to function well in practice. The purposes of this study were (1) to measure psychological assessment competency (PAC) for master’s level interns and (2) to investigate the current psychological assessment education and training in Taiwan. Thirty dyads of clinical psychology interns and their supervisors were recruited as participants for the study. They were assessed with a questionnaire (including the internship version of the Psychological Assessment Competency Scale (PACS-I) and other questions) at the beginning and the end of internship. The PACS-I required both interns and supervisors to evaluate the interns’ competencies in psychological assessment. The PACS-I included 15 5-point Likert scale items grouped into three domains: knowledge of psychological testing, assessment skills, and interpersonal relationship. The results indicated that interns at the beginning of the internship considered themselves having the best performance on the interpersonal relationship domain, and their performance on the other two domains would require further enhancement. At the end of the internship, interns considered themselves to have significantly improved on all three PACS-I domains. A similar pattern was shown on supervisors’ post-internship PACS-I ratings of their interns in comparison to the pre-internship ones. To maximize the internship training effects, supervisors suggest that interns meet the following four basic competencies before entering internship: three PACS-I domains and knowledge of differential diagnosis. Due to the small sample size and most participants being recruited from the division of psychiatry in the hospital, the findings of this study may not generalize to other training sites.