Fine structures appearing on the phase transition fromh(hexagonal) toc(cubic) boron nitride under high pressure (7.7 GPa) and high temperature (1800–2150 °C) are examined by high‐resolution transmission electron microscopy. A prominent contraction of the interplanar spacing betweensp2sheets from 3.33 to 3.10 A˚ in so‐called ‘‘compressedh‐BN’’ is attributable to a monoclinic lattice distortion of the residualh‐BN, which originates from the difference in the compressibility as well as the thermal expansion between adjoiningh‐ andc‐BN grains. The parameters of the monoclinic unit cell aream=4.33,bm=2.50,cm=3.1–3.3 A˚, and &bgr;=92–95°. Thin plates ofh‐BN are often folded and the folding also causes the monoclinic structure. The sheet sequence ofr(rhombohedral)‐BN locally appears when the strong volume shrinkage occurs due to the formation of ac‐BN grain. Nanoscale twins appear in resultingc‐BN grains, as long as they are small, andw(wurzite)‐BN is sometimes included in them. ©1996 American Institute of Physics.