Abstract- In this paper we investigated theoretically an enhancement in optical absorption and current density in organic solar cell with finite different time domain(FDTD) method, due to the light trapping in grating structure .we show that there is an optical absorption and current density enhancement ,with TiO2 grating structure, with a special thickness and period, in top electrode, due to the diffraction and light path enhancement within solar cell. We show also optical absorption and current density enhancement according to the thickness and period, and we will determine an optimum thickness and period for absorption and current density enhancement.