A linearly graded doping drift region structure, a novel lateral voltage-sustained layer used for improvement of reduced surface field (RESURF) LDMOS transistor performance has been evaluated theoretically, numerically and experimentally in this paper for the first time. Due to the coupling effect of the two-dimensional (2D) electrical field, it is found from the theory developed here that the linearly graded drift region-doped profile can provide a high breakdown voltage while maintaining a high doping dose in the total drift region for minimizing the on-resistance Ron. The characteristics of such an LDMOS have been demonstrated by the 2D semiconductor device simulator MEDICI and further verified by our experimental results. We have obtained a reduction of the on-resistance of 50% from 10.3 mΩ cm2 to 5 mΩ cm2 in the on-state, and an increase of the breakdown voltage by a factor of 2.5 from 90 V to 234 V in the off-state, compared to the values for conventional RESURF devices. The experimental results verify the performance improvement predicted by the simulation and theory.