The growth mechanisms and the relaxation of the lattice parameter mismatch was investigated for the growth of In0.6Ga0.4As layers on GaAs(001) substrates. Layers with a nominal thickness between 2 nm and 1 mu m were grown by molecular-beam epitaxy and characterized by plan-view transmission electron microscopy, high-resolution transmission electron microscopy on cross-section samples and X-ray diffractometry. The growth process can be subdivided into the following three stages : formation of a coherently strained Stranski-Krastanow layer; relaxation of individual islands by misfit dislocations; and coalescence of islands leading to a two-dimensional growth mode for thick layers. A quantitative analysis of the lattice parameter from high-resolution electron microscopy images is presented. It is shown that the misfit relaxation in islands is accommodated by the generation of misfit dislocations plastic relaxation) and by the distortion of lattice planes (elastic strain relaxation). A large residual strain is observed al the interface of thick layers which is reduced with increasing distance from the interface.