This study was conducted in order to understand the implementation quality of the Secondary 1 Program of the Tier 1 Program of the Project P.A.T.H.S. (Positive Adolescent Training through Holistic Social Programmes) in the first year of the Full Implementation Phase. Classroom observations of 137 units in 85 schools were conducted under the Co-Walker Scheme. Results showed that the overall level of program adherence was generally high, with an average of 86.57%. Thirteen aspects concerning program delivery were significantly correlated. Multiple regression analyses revealed that (1) overall implementation quality was significantly predicted by interactive delivery method, use of positive and supportive feedback, opportunity for reflection, degree of achievement of the objectives, and lesson preparation; whereas (2) success of implementation was significantly predicted by student interest, interactive delivery method, use of positive and supportive feedback, opportunity for reflection, and degree of achievement of the objectives. In general, the present study suggests that the implementation quality of the Project P.A.T.H.S. is good.