The formation of BN in a 9Cr3W3CoB steel during high temperature aging was investigated. It was found that majority of B was presented as M₃B₂ borides in the as-treated  steel. Due to the low content of N and the existence of M₃B₂ borides, only a few BN were distributed along PAGBs. Metastable M₃B₂ borides would gradually dissolve during isothermal exposure, leading to the precipitation of BN. The nucleation and growth of BN were thermodynamic processes affected by both aging time and temperature. Its number density and size increased with increasing the temperature. During aging at 800 ℃, BN nucleated quickly and transferred from subsphaeroidal shape to irregular shape with sharp corners, and this kind of BN particles with sharp corners was detrimental to the toughness. Thermo-mechanical treatment was employed to eliminate the harmful M₃B₂ and BN precipitates in the  steel and hinder the nucleation and growth of BN at 800 ℃.