One of the big challenges by molecular theory is understanding the fracture process of semicrystalline polymers, which is important to increase toughness of polymeric materials in industry. The fracture process against the stretching is also tempting in physics because semicrystalline polymers show inhomogeneous structure and the process takes non-equilibrium pathway. In my talk, the fracture process of polyethylene is studied by coarse-grained molecular dynamics simulation. Lamellar layer, which consists of amorphous and crystal parts and is basic structure of polyethylene in molecular scale, is stretched parallel and perpendicular to the crystal direction. In the stretching process, recrystallization of grain boundary, fragmentation of lamellar structure, and tilt of crystal part are observed. Dynamics of polymer chains in molecular scale are successfully revealed.