The microstructure of HTS YBa2Cu307 _ x single crystals with x 1> 0.5 has been investigated by TEM and selected area diffraction. An inhomogeneous oxygen distribution over the crystal was observed. Large differences between the bulk crystal structure and the surface have been established. The bulk structure was presented by orthorhombic blocks (~ 100 x 10 × 20nm in sizelembedded in tetragonal matrix with x-~ 1. The bulk structure was not of a superconductive type. The crystal surface was enriched by oxygen and had the usual orthorhombic twinned structure. The superconductive properties of the whole specimen were determined by surface layer about 5/~m thick.