The present invention relates to a method of fabricating a high-voltage high-power integrated circuit device using a substrate of a SOI structure in which an insulating film and a silicon layer are sequentially stacked on a silicon substrate. The method comprising the steps of sequentially forming an oxide film and a photoresist film on the silicon layer and then performing a photolithography process using a trench mask to pattern the photoresist film; patterning the oxide film using the patterned photoresist film as a mask and then removing the photoresist film remained after the patterning; etching the silicon layer using the patterned oxide film as a mask until the insulating film is exposed to form a trench; forming a nitride film on the entire surface including the trench, performing an annealing process and depositing polysilicon on the entire surface so that the trench is buried; and sequentially removing the polysilicon and the nitride film until the silicon layer is exposed to flatten the surface, thus forming a device isolating film for electrical isolation between devices within the trench. Therefore, the present invention can effectively reduce the isolation area of the trench between the high-voltage high-power device and the logic CMOS device and can easily control the concentration of a deep well.