The curvature effect of the 3-D layout may result in a severe deterioration of the breakdown voltage in a power lateral double-diffused MOS (LDMOS). In this paper, a novel 3-D analytical model is proposed to provide the physical insight of curvature effect in small radius region by solving the 3-D Poisson in cylindrical coordinates. The proposed model indicates that the curvature effect equivalently changes the drift region doping concentration, thus altering the surface electric field profile. The proposed 3-D model can accurately describe the surface field profile in the fingertip regions of the multifinger layout, and provides an effective way to reveal the influence of curvature effect on the performance of silicon-on-insulator LDMOS. Furthermore, based on the proposed model, a corresponding structure optimization criterion is derived to provide a simple but effective method for the device structure parameters optimization.