The strain effects in and the crystal structures of self-assembled InAs/GaAs quantum dots (QDs) were investigated by using transmission electron microscopy (TEM). The in-plane lattice constant of the InAs QDs was larger than that of the GaAs substrate, and the vertical lattice constant of the InAs QDs was smaller than that of the InAs bulk. The variation of the lattice constant for the InAs QD originated from the strain effect. A schematic diagram of a strained InAs QD based on the TEM results, indicative of the strain distribution around the QD, is presented. (C) 2003 American Institute of Physics.