A thermally conductive sheet comprising boron nitride powder as a first thermally conductive filler dispersed into an organic matrix, wherein the boron nitride powder is hexagonal boron nitride (h-BN), and comprises primary particles and secondary aggregates formed by aggregation of the primary particles, wherein at least part of the secondary aggregates being 50 µm or more in size. The boron nitride powder preferably includes 1 to 20 percent by weight of secondary aggregates of 50 µm or more in size. The organic matrix material is preferably silicone rubber.