Provided is a boron-nitride powder that is suitable for use in a resin composition for transmitting heat from a heat-producing electronic component such as a power device to a heat-dissipating member. The boron-nitride powder reduces thermal-conductivity anisotropy and thermal contact resistance, resulting in high thermal conductivity, and contains boron-nitride particles each consisting of hexagonal boron-nitride primary particles joined together. The boron-nitride powder, which is an aggregate of said boron-nitride particles, exhibits a mean sphericity of at least 0.70, a mean particle diameter of 20-100 μm, a porosity of 50-80%, a mean pore diameter of 0.10-2.0 μm, a maximum pore diameter of at most 10 μm, and a calcium content of 500-5,000 ppm. Under X-ray powder diffraction, the graphitization index of the boron-nitride powder is preferably between 1.6 and 4.0, inclusive, and the peak intensity ratio (I(002)/I(100)) between the (002) plane and the (100) plane is preferably at most 9.0.