Abstract: A two-dimensional relativistic electromagnetic particle simulation code (2DCIC) is presented. The charge and current densities are computed self-consistently using Maxwell's equations, relativistic motion equations of electrons and Newton's equations of ions by tracking 10⁴-10⁶ simulated particles. some details of wave wave and wave particle interaction are also studied together with the evolution of instability as time. Laser light impinges obliquely (or normally) on plasma, which has a density gradient (or homogeneous density). To save computational time, the parallel computational method is developed. Using this code, the ultrashort intense pulse laser plasma interaction and other plasma problems can be also studied by modifying parameters of physical model. After a lot of test computation, plasma equilibrium and propagation of ultrashort intense pulse laser in plasma are studied. The computed physical picture is reasonable.