Misfit relaxation by dislocations in perovskite SrRuO3/SrTiO3 (001) heterostructure with low lattice mismatch (f=0.64%) was studied. Pure edge misfit dislocations (MDs) with a Burgers vector of the a<011> type were found to be the major interfacial defects responsible for the misfit relief. They were introduced by half-loops expansion from the film surface as well as by extension of pre-existing dislocations in the substrate. These 45°-MDs formed along the <100> directions in a rectangular grid pattern, preferentially at surface steps of the TiO2-terminated SrTiO3 (STO) substrate. At film thicknesses much higher than the critical film thickness (hc), however, the MD spacing was not reduced but saturated in a nearly constant value far above the equilibrium prediction (∼61.4 nm). The saturated spacing of MDs corresponds to roughly the ledge distances on the STO surface (120–150 nm). It is suggested that difficulties in the multiplication of MDs by the already-formed ones lead to the abnormally high residual strain. Further accumulation of residual strain results in microstructural modifications such as surface undulations and the two-layered structure along the growth direction. © 2004 American Institute of Physics.