A method for manufacturing a lateral double diffused metal oxide semiconductor (LDMOS) device includes forming an oxide layer on a semiconductor substrate, forming first and second trenches by partially etching the oxide layer and the semiconductor substrate, forming a small trench overlapping with the second trench so that the second trench has a stepped structure, and depositing one or more dielectric layers so that the first trench forms a device isolation layer defining a semiconductor device region and the second trench having a stepped structure forms a drain extension device isolation layer. The breakdown voltage of the LDMOS device may be improved while reducing the on-resistance, thereby improving the operational reliability of the device.