The existing form of BN particles, a new environmentally friendly free cutting particle, is very important in steel for the purpose of optimizing cutting performance. However, precipitation and growth behaviors of BN inclusions in steel are not very clear until now. In this research, the morphologies of BN inclusions in steel were discussed and the formation mechanisms of BN inclusions were proposed. First, the precipitation behaviors of BN inclusions in steel have been investigated by theoretical calculation using Factsage software. Then, the actual three-dimensional morphology of BN inclusions and composite inclusions were observed in laboratory. Moreover, Growth model of BN inclusions via diffusion and collision are established employing Matlab, respectively. The results revealed that BN precipitate greatly in the solidification end and BN can nucleate heterogeneously on the surface of nucleation site (such as Al2O3, spherical MnS) besides homogeneous nucleation. The size of the BN ranged from 1 to 2 um after the diffusion growth, and then BN particles and other particles in steel, relying on its own Brownian motion, collided and agglomerated with each other, thereby formed a large cluster-like inclusion. The results agreed well with the simulated results based on BN growth model.