A method for establishing a scattering bar rule for a mask pattern for fabricating a device is provided. The method is described as follows. First, at least one image simulation model is established according to the mask pattern and a process reference set used for fabricating the device based on the mask pattern. Next, a plurality of scattering bar reference sets is applied to the image simulation model so as to generate a plurality of simulation images, respectively. Further, a portion of the simulation images are selected to be a plurality of candidate layouts according to a screening criterion. Next, one of the candidate layouts is determined to be a pattern layout according to a selection rule, and the scattering bar reference set corresponding to the pattern layout is determined to be a scattering bar rule of the mask pattern.