This study tested how collaboration, as a component of school capacity and a potential mediator, affect the relationship between principal leadership and teacher professional learning in Hong Kong primary schools. To investigate teacher perceptions of principal leadership and school capacity, two questionnaires were combined to use. The online survey was administered with a sample of 970 teachers from 32 primary schools. The orthodox regression based dour-step causal approach (Baron & Kenny, 1986) integrated with the bootstrapping method, was adopted for mediating analysis, with the composite score of principal leadership as the predictor. Upon the affirmation of the presence of the mediating effects, significance and strength were further assessed. This study stepped further to test the overall effect of the seven core areas of principal leadership, as independent predictors, on teacher professional leaning via the possible mediating effects of collaboration. The findings affirmed that, collaboration did play a mediating role between the relationship of principal leadership and teacher professional learning in Hong Kong primary schools.