The relaxation process in InAsP/InGaAsP strained-layer superlattices (SLSs) with interfacial misfit dislocations has been investigated systematically by transmission electron microscopy (TEM) and x-ray analyses. The TEM analysis reveals that dislocations locate a little inside the buffer layer near the interface between the buffer and first well layer in the SLS. The x-ray analysis of (400) azimuthal angle dependence indicates the buffer layer has a large macroscopic tilt. Using a curve fitting analysis of various(hkl)x-ray profiles and reciprocal lattice mapping measurements, residual strain was determined quantitatively, i.e.,&Dgr;a∥and&Dgr;a⊥,in the SLS and buffer layer. These results reveal that the dislocations mainly cause lattice distortion of the buffer layer rather than relaxation of the SLS layer. The most remarkable result is that the change ofa∥is not equal to that ofa⊥in the buffer layer. This phenomenon strongly suggests that microplastic domains are generated in the buffer layer. ©1997 American Institute of Physics.