The compatibility of isotactic polypropylene and ethylene-1-butene copolymers changes with the content of 1-butene in the copolymer so that the morphology of their blends changes drastically. The relationship between morphology and mechanical properties for blends of isotactic polypropylene and ethylene-1-butene copolymer are investigated using a rheo-optical technique; the infrared absorbance spectroscopy and the stress are measured simultaneously under a constant rate of elongation. It is found that the compatible blends affinely deformed accompanied by little plastic deformation such as microvoids and craze, whereas the incompatible blends occurring a phase separation cause a segregation at interface between the two phases in the low strain region. (C) 1997 Elsevier Science Ltd.