Coarse-grained molecular dynamics simulations are conducted, for the purpose of the study on the microscopic structure change of crystalline polymer due to degradation. Interlamellar structure changes are observed during the degradation, decreasing loop and bridge chains, increasing short tail chains. From the elongation simulation of lamellar structure, craze strength is dropped during the degradation, because of decreasing oriented interlamellar chains and drawing of the molecular chains from the crystal part. Multi-scale modeling is conducted to demonstrate the relationship between microscopic structure changes and macroscopic mechanical properties. As a result, macroscopic fracture strain is reduced by the degradation, because craze growth rate becomes faster with the degradation.