This study attempted to identify effects of trust between principal leadership and teacher professional learning in Hong Kong primary schools. To verify the potential mediating effects of trust as a component of school capacity, survey data with a sample of 970 teachers from 32 local primary schools was used. Two questionnaires were combined to investigate principal leadership and school capacity in the schools. Baron and Kenny's four-step causal process for mediation analysis was employed to have a preliminary inquiry. To compensate weaknesses of the orthodox regression based approach, significance of the mediating effects of trust were tested using the Sobel's test and bootstrapping method. Next, the seven core sets of principal leadership practices were used as multiple predictors, and mediating effects of trust between them and teacher professional learning were also examined. The findings affirmed the role of faculty trust as a mediator between principal leadership and teacher professional learning in Hong Kong primary schools.