Abstract: The propagation of shock wave and it's reflection at free surface in two-dimensional lattice are considered and calculated by method of molecular dynamics. This lattice simulates the (1,0,0) plane for face centered cubic (f.c.c) or the (1,1,0)plane for body centered cubic(b.c.c).In the range of our calculation (u_p=10-75×10⁴cm/sec), the oscillation of particle velocity is'nt decrescent. The shock wave velocity is a linear function of average particle velocity. After reflection, the average particle velocity near free surface is equal to twice the piston velocity approximately, which agrees with macroscopic phenomena. The particles at outer-most layer of free surface, get the large value of velocity and go away from this surface. It is also similar to the macroscopic phenomena of ejection. At the later stage of expansion, the particles in the interial region may lose connection between them, that corresponds to the microscopic fracture.

Key words: Molecular Dynamics, shock wave, Lattice