Dimension effect on breakdown voltage (BV) of lateral double-diffused metal-oxide–semiconductor field-effect transistor in partial silicon-on-insulator (PSOI) technology is comprehensively studied. The maximum BV (BV_max) is examined under various settings of the device length ${L}$ and the active silicon film thickness ${t}$ . It is shown that there exists an optimal pair of ( ${L}$ , ${t}$ ) for PSOI at which the highest BV can be achieved. The ratio of ${L}$ / ${t}$ is better chosen between 5 and 7 for the device designs, in particular, $L/t={\mathrm{ 6}}$ can be considered as the optimal one theoretically. Moreover, impacts of the silicon window length $L_{w}$ and the drift doping concentration $N_{dr}$ on the BV, the on-resistance ( $R_{\mathrm{ on}}$ ) and the figure-of-merit ( $=\!{\mathrm{ BV}}^{2}/R_{\mathrm{ on}}$ ) are also carefully studied.