There are only a few process evaluation studies on positive youth development programs, particularly in the Chinese context. Objectives: This study aims to examine the quality of implementation of a positive youth development program (Project Positive Adolescent Training through Holistic Social Programs [P.A.T.H.S.]) and investigate the relationships among program adherence, process factors, implementation quality, and success. Method: Process evaluation of 20 Secondary 3 classroom-based programs was conducted in 14 schools. Results: Overall program adherence, individual evaluation items, quality, and success had high ratings. Principal components analysis showed that two components, namely, implementation process and implementation context were extracted from 11 evaluation items. The correlational analysis indicated that program adherence, implementation process, and context were highly correlated with quality and success. Multiple regression analyses show that teaching process and program adherence predicted quality, whereas teaching process, teaching context, and program adherence predicted success. Conclusions: The implementation quality of the Tier 1 Program of Project P.A.T.H.S. was generally high. (Contains 6 tables.)