Abstract: A method of forming a phase shift mask is disclosed. After providing a substrate with a shifter layer and a Cr layer, a first photoresist layer is formed on the Cr layer. Thereafter, a position of a shifter pattern is defined by using E-beam writing process and then a develop process is performed. After that, a first Cr pattern is formed by using an etching process to etch the Cr layer. After removing the first photoresist layer, the shifter pattern is formed by using an etching process to etch the shifter layer. After forming a second photoresist layer, an exposure process of conventional photolithography is performed by means of a boarder mask to define a position of a second Cr pattern. After performing a develop process, the second Cr pattern is formed by using an etching process to etch the first Cr pattern.