This study examined the implementation quality of the Tier 1 Program (Secondary 1 Curriculum) delivered in the second year of the Full Implementation Phase of the Project P.A.T.H.S. (Positive Adolescent Training through Holistic Social Programmes). Under the "Co-Walker Scheme", systematic observation of curriculum units was conducted in 138 schools. Results indicated that the overall level of program adherence was high, with an average of 82.9%. The mean ratings of the program implementation quality were also high. Despite limitations, the findings of this study suggest that the implementation of the Secondary 1 Program (Tier 1 Program) of the Full Implementation Phase was of very high quality. The present findings also provide strong evidence to account for the successful and encouraging outcomes of a major positive youth development program in Hong Kong. Copyright © 2009 with author. Published by TheScientificWorld.