This work is dedicated to design a power SOI (Silicon-On-Insulator) MOSFET switch to replace the Mechanical power relays used in Electrical systems. Mechanical relays are relatively large in size, expensive & have slow response time. Presented simulations & measurements show that two bulk MOSFETs in series can replace a mechanical relay in an AC drive, however, losses increase due to the series connection. Indeed, SOI power switch becomes to have better characteristics over the mechanical or Bulk MOSFET power switches in terms of higher breakdown voltage, lower specific on-resistance, low leakage current and the controllability of heat impact. Many SOI technologies are adopted for power and automation applications and the most intriguing power SOI is found to be the RF LDMOSFET. Two dimension Numerical Simulations has been achieved using Medici tool. RF LDMOSFET exhibits high breakdown voltage of 70 V and low specific on-state resistance of 0.19 mΩ cm2.