Crystallization behavior of synthetic cis-1,4 polyisoprene mixed with methyl linolate was investigated by differential scanning calorimetry and dilatometry. At isothermal crystallization temperature of -25 degrees C, a rate of crystallization decreased with the addition of 1 wt % methyl linolate, whereas it increased with a large amount of methyl linolate, about 30% by weight. Although Avrami constant of cis-1,4 polyisoprene was 2.82, that of mixtures containing 1 and 30 wt % methyl linolate was found to be approximately 4. This implies the same crystallization mechanism was in operation in the mixtures. Since glass transition temperature, T-g, of cis-1,4 polyisoprene decreased as the methyl linolate content of mixture increased, methyl linolate was found to be plasticizer for cis-1,4 polyisoprene. The increase in the rate of crystallization in the mixtures was corresponding to the decrease in T-g.