In this paper，a novel analytical model for breakdown voltage of SD LDMOS is developed. Based on the 2-D Poisson's solution，the model gives the analytical solutions of the surface potential and electrical field distributions as functions of the structure parameters and drain bias；and the dependence of breakdown voltage on structure parameters is calculated. An effective way to attain the optimum high-voltage devices is also proposed. All analytical results were verified by simulation results obtained by MEDICI and previous experimental data，showing the validity of the present model. As a result of the modulation of the drift region and subtrate' field，the on-resistance is decreased and new electrical field peak are produced inside the drift region which improves the surface electrical field distribution and increases the breakdown voltage. The SD structure led to a significant improvement of breakdown voltage of about 26% and a reduction of on-resistance of about 33% compared to the conventional structure.