There is more probability that the defects exist in the specimen and the tensile strength decreases, as the size of the specimen becomes larger. This is a phenomenon called 'scale effect', but generally this effect has been only experienced and discussed for the brittle materials. The authers investigated this effect of the high polymer on the viscous fracture with a large deformation. It is supposed to be difficult to treat the brittle material and the viscous material in a similar way. We applied K. Fujino's statistical treatment method to our study. Fujino's statistical treatment is based on the theory of probability that the probability of the existence of the weak defects increases as the length of thread becomes longer. In this paper the scale effect has been investigated for the high polymer through the tension tests and the theory of probability has been applied successfully to the viscous fracture.