Technology Computer Aided Design (TCAD) for the laterally diffused MOSFET (LDMOS) using ultra-thin body and buried oxide (BOX) silicon-on-insulator (UTBB SOI) is demonstrated. The sustainability of high voltage of LDMOS makes it useful for high-voltage input-output devices (I/O devices). With the back-gate bias technique, three distinct biases used for core devices are also applied to I/O LDMOS for design simplicity. This work shows that electric field and series resistances in the drift region can be modulated by back-gate bias for I/O performance improvement.