The machinability and deformation mechanism of Si3N4/BN nanocomposites were investigated in the present work. The fracture strength of Si3N4/BN microcomposites remarkably decreased with increased hexagonal graphitic boron nitride (h-BN) content, although machinability was somewhat improved. However, the nanocomposites fabricated using the chemical method simultaneously had high fracture strength and good machinability. Hertzian contact tests were performed to clarify the deformation behavior by mechanical shock. As a result of this test, the damage of the monolithic Si3N4 and Si3N4/BN microcomposites indicated a classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.