Optical force was calculated for a double-layer dielectric sphere. Results indicate that the optical force was determined by the relative refractive index and thickness of the two layers. Especially, when the refractive index of the inner layer is higher than that of the outer layer, a smaller region, where the trapping stiffness is improved effectively, can be achieved with the reducing of the radius of the inner layer, and the double-layer dielectric sphere can be confined stably in this region. In comparison, if the index of refraction of the inner layer is lower than that of the outer layer, two stable equilibrium positions may be found when the inner radius decreases.