Effect of side methyl and dimethyl groups of soft segment component on polyurethane elastomers (PUEs) was investigated with and without elongation. The polymer glycols used were poly(oxytetramethylene) glycol (PTMG), PTMG with dimethyl groups (PTG-X) and methyl groups (PTG-L). Phase separation of the PUEs became weaker with an increasing methyl group content. Tensile test revealed that the increasing methyl group concentration made the PUEs be soften and weaken. The PTMG based PUEs obviously exhibited elongation-induced crystallization during elongation process. In contrast, for the PTG-L and PTG-X based PUEs, crystallinity decreased with an increasing side methyl group content, and the PUEs with PTG-L and PTG-X with highest methyl group content did not crystallize. We succeeded the control of the crystallization behavior of elastomers under an elongation by the introduction of side methyl groups. (C) 2004 Elsevier Ltd. All rights reserved.