A new semianalytical method, which is a combination of the density functional theory with Rosenfeld density functional and the Ostein-Zeike equation, is proposed for the calculation of the effective depletion potentials between a pair of big spheres immersed in a small hard sphere fluid. The calculated results are almost identical to the integral equation method with the Percus-Yevick approximation, and are also in agreement well with the Monte Carlo simulation results.