A method for fabricating a semiconductor power integrated circuit includes the steps of forming a semiconductor structure having at least one active region, wherein an active region includes a well region for forming a source and a drift region for forming a drain region, forming a trench for isolation of the active regions, wherein the trench has a predetermined depth from a surface of the semiconductor structure, forming a first TEOS-oxide layer inside the trench and above the semiconductor structure, wherein the first TEOS-oxide layer has a predetermined thickness from the surface of the semiconductor device, forming a second TEOS-oxide layer on the first TEOS-oxide layer, wherein a thickness of the second TEOS-oxide layer is smaller than that of the first TEOS-oxide layer, and performing a selective etching to the first and second TEOS-oxide layers, to thereby simultaneously form a field oxide layer pattern, a diode insulating layer pattern and a gate oxide layer pattern, to thereby reduce processing steps and obtain a low on-resistance.