We have investigated the effect of micro-twin defects on InSb quantum wells (QWs) grown on GaAs (001) substrates. Transmission electron microscopy analysis revealed that the QW layers are intersected by micro-twins that arise from lattice mismatch with the substrate. The intersected parts of the QWs lie near or in the {111} plane, which is tilted by 15.8 degrees with respect to the (001) substrate. Hall effect measurements indicated that the degradation of electron mobility in the QWs is well correlated with the density of the micro-twins. 0 2005 American Vacuum Society.