Boron nitride and its production

The present invention relates to a boron nitride having the following features and relates to its production: (1) preparing a hexagonal boron nitride polycrystalline consisting of microcrystals having a nano-order level, (2) hexagonal boron nitride microcrystalline powder having 20-200nm average grain diameter and containing ≥80 wt.% grains in the range of (the average grain diameter)±30%, (3) a fin h-BN powder of grains each having a prolate spheroid-like or circular disk-like shape with the minor axial length La, major axial length Lb and thickness t satisfying 50 nm ≤ La ≤ Lb ≤ 400 nm, 20 nm ≤ t ≤ 150 nm, t ≤ La, and 0.5 ≤ La/Lb ≤ 1.0.