AbstractWe propose a new method of calculating elastic modulus and atomic displacements of a stretched polymer chain. We use a stretched Cartesian coordinate system and a stretched B matrix defined by the Cartesian displacement components of the bonds to express the potential energy matrixFxfor stretching. The method is used to calculate elastic moduli of planar‐zigzag‐chain vinyl polymers and helical‐chain isotactic polypropylene and the atomic displacements of their skeleton and side groups. The skeletal displacements of planar‐zigzag‐chain polymers are about ten times larger than those of the side groups and reverse displacements of the two adjacent skeletal units occur in the direction perpendicular to the stretching direction along the fiber axis. In the case of isotactic polypropylene, the methyl group has more effect than the torsional change around the CC bond on the skeletal deformation. © 1993 John Wil