Machinable silicon nitride/ hexahedral boron nitride (Si3N4/h-BN) composites were in-situ synthesized in a nitrogen (N2) atmosphere by means of combustion synthesis gas-solid reaction with silicon (Si) powder and h-BN as raw materials. The effect of the volume fraction of h-BN on the machinable properties of Si3N4/BN composite was studied. The results show that Si powder was fully nitrified and no residual Si was found. Microstructures by a scanning electron microscopy (SEM) show Columnar crystals of β-Si3N4 are the main phase and acicular crystals of h-BN disperse β-Si3N4 intergranular. With the increasing of the volume content of h-BN, the machinability of the composite increases, but the bending strength of composite decreases firstly and then increases. The lowest bending strength is 84.96MPa at 25% volume fraction of h-BN.