Abstract: The dependence of viscoclastic properties on the molecular weight and MWD of cis-1,4-polyisoprene has been studied with the methods of GPC and stress relaxation. The results show that the relaxation modulus and the maximum relaxation time of Ln-PIR depend not only on the molecular weight, but also on the MWD.The value of K chan-ges with MWD, but the β is essentially constant in the relationship of τ_m=KM_w^β. Higher chain entanglement density leads to the non Newtonian effect of Ln-PIR.Hence,the relationship between τ_m and M_w deviates from the 3.4th power rule. This may be corrected with the density value of chain entanglement. The low molecular weight fractions act as a diluent in the chain entanglement net-works. The samples with broader MWD have higher critical molecular weight M_c of entanglement networks.