This study tests mediated principal leadership effects on teacher professional learning through collegial trust, communication and collaboration in Hong Kong primary schools. It is based on a series of single mediator studies, and uses the same convenience sample of 970 teachers from 32 local primary schools. It also adopts regression-based macros, integrated with bootstrapping, to examine and compare sizes and proportions of potential mediating effects of the three human relational variables. The findings affirm the role and nature trust, communication, and collaboration play in the mediated relationship. In contrast, the mediating power of collaboration is non-significantly stronger than that of communication, and is more than double that of trust. The conclusion is that a school environment featuring mutual trust, effective communication and genuine collaboration is a core condition for teacher learning and change. Provided that the forces that bind people together in schools are multiple, principals are recommended to create school culture and conditions strategically for teacher learning to thrive.