Abstract: An SOI LDMOS device structure with Oxide By-passed（OB） was investigated and its breakdown mechanism and characteristic of structure was analyzed.Its performance was verified by three dimensional numeric simulation with SILVACO TCAD software.The simulated results show that the electrical field element of the device is modulated by the technology of similar Superjunction（SJ） structure.Compared with the SJ LDMOS device,OB LDMOS achieved not only the same breakdown voltage,but also the specific on-resistance of the OB LDMOS reducing from 3.81mΩ·cm² to 1.96mΩ·cm²,except for achieving comparable performance and overcoming the high aspect ratio of fabrication structure and difficulty of accurate concentration match of SJ LDMOS.