A prominent contraction of the interplanar spacing between sp2 sheets from 3.33 to 3. 10Å in so-called “compressed h-BN”, which appears on the phase transition from h(hexagonal)- to c(cubic)-boron nitride (BN) under high pressure (7.7GPa) at high temperature (1800°C), is examined by high-resolution transmission electron microsco)py (HRTEM). It is attributable to a monoclinic lattice distortion of the residual h-BN, which originates from the difference in the compressibility as well as the thermal expansion between adjoining h- and c-BN grains. The parameters of the monoclinic unit-cell are am=4.33, bm=2.50, cm,=3.1–3.3Å and β=92 - 95°. Thin plates of h-BN are often folded and the folding also causes the monoclinic structure.