A surface-treated aggregated boron nitride powder is prepared by using the boron nitride powder as the raw material, adding an oxidizer to the boron nitride aggregated grains, wet-pulverizing or wet-crushing the grains for surface modification treatment of the particles and allowing reaction of the particles with a metal coupling agent. The surface-treated boron nitride aggregated grains are formed by aggregation of hexagonal h-BN primary particles; (B) have any one or more of Si, Ti, Zr, Ce, Al, Mg, Ge, Ga, and V in an amount of 0.1 atm % or more and 3.0 atm % or less in its composition on the surface of 10 nm; (C) have a crushing strength of 5 MPa or more; and (D) have an average particle diameter of 20 μm or more and 100 μm or less.