A method of forming a power MOSFET is described. An epitaxial layer of first conductivity type is formed on a substrate of first conductivity type. A body layer of second conductivity type is formed in the epitaxial layer. A plurality of mask patterns are formed on the substrate. A plurality of trenches are formed in the body layer and the epitaxial layer between the mask patterns. An oxide layer is formed on surfaces of the trenches. A conductive layer is formed in the trenches. A trimming process is performed to the mask patterns to reduce the line width of each mask pattern. Two source regions of first conductivity type are formed in the body layer beside each trench by using the trimmed mask patterns as a mask. A plurality of dielectric patterns are formed on the conductive layer and between the trimmed mask patterns. The trimmed mask patterns are removed.