The temperature and magnetic field dependences of the critical current density in silicone oil doped MgB2 polycrystalline samples have been investigated by magnetic measurements. The upper critical magnetic field anisotropy, γ=H_c2^(⟘c)/H_c2^(||c), dependence of the critical current density, Jc(B), were analyzed within the percolation model . The calculated critical current densities based on percolation theory are in agreement with the experimental data. It was found that the anisotropy, γ, and the percolation threshold parameter, pc, show different trends in their temperature dependence, where γ increases, but pc decreases with increasing temperature. It is suggested that the anisotropy is responsible for the reduction of the critical current density in high magnetic field. The relationship between the anisotropy and the volume pinning force is investigated. It was found that the position of the maximum of the volume pinning force is shifted to lower reduced magnetic field by decreasing the anisotropy and increasing the percolation threshold pc.

Keywords