The structural origin of rubber elasticity in the polyolefinic thermoplastic elastomers composed of isotactic polypropylene (iPP) matrix and ethylene - propylene - diene rubber (EPDM) domains was investigated using scanning and transmission electron microscopes under uniaxial deformation and the computational analysis by a three dimensional finite element method. The rubber domains were dominantly deformed and elongated by accompanying localized yielding in iPP region between neighboring EPDM domains perpendicular to the stretching direction. The iPP region between adjacent EPDM domains in the stretching direction remained undeformed, suggesting that the undeformed iPP region plays the role in connecting rubber domains. (C) 2004 Elsevier Ltd. All rights reserved.