In an attenuated phase-shift mask (PSM) and a method of forming the same, a phase-shift layer and a light-shielding layer are sequentially stacked on a transparent substrate. The phase-shift layer and the light-shielding layer are sequentially removed from the substrate, to form a light-shielding pattern including a first opening and a phase-shift pattern including a second opening that is connected to the first opening and partially exposes the transparent substrate. Then, a transmitting portion is formed through the light-shielding pattern by partially removing the light-shielding pattern. The transmitting portion includes at least one portion of the phase-shift pattern on which a transmittance controller is formed. In one embodiment, the transmittance controller comprises a metal having a high absorption coefficient, and is formed through sputtering and diffusion processes. Accordingly, the intensity deviation between 0th and 1st order beams may be decreased, to thereby improve the processing margin of the exposure process.