Quaternary GaInAsSb epilayer was grown by MOCVD on GaSb substrate, and the crystalline state and mismatch relaxation are studied by scanning transmission electron microscopy (STEM), bright field images and convergent beam electron diffraction (CBED) patterns. Mismatch dislocations are generated from the interface, and Lomer 90 degrees dislocations induce surface ridges, and the shift of Kikuchi lines can determine relaxation quantitatively which is similar to the value calculated from electron probe microanalysis.