Microstructures of three types of YBa/sub 2/Cu/sub 3/O/sub 7- delta / single crystals having strong differences in their j/sub c/ levels have been studied by means of electron microscopy. In order to find any existing relation between the microstructures and their j/sub c/ values, histograms of the frequency of twin spacings were obtained from SEM as well as TEM measurements. These histograms show that the finest and most regular twin domain texture (twin spacing 130+or-80 nm) may be correlated with the highest j/sub c/ values. The single crystals with the highest j/sub c/ values have stacking faults (due to double Cu-O chain layers) homogeneously distributed in an otherwise rather perfect matrix of 123-phase. These stacking defects have a limited extension of approximately 80 nm, while in the other types of crystals, they have an extension approximately 1 mu m. These results suggest that relatively higher j/sub c/ values are associated with certain characteristic microstructural features related to twin domain boundaries as well as to stacking faults.