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Line edge roughness (LER) has become a standard topic in the semiconductor industry for its possible yield impact on wafer production. Recently a number of studies address its measurement process to increase the reliability of results. Here, we investigated roughness on photo lithographic masks, which is the blue print for wafer production. It is shown that LER influences feature uniformity and thus has a considerable impact on overall mask production yield. To determine the roughness parameters we varied measurement parameters on a scanning electron microscope to optimize reproducibility and repeatability of the obtained values. Two parameters dominate the LER values. The first parameter is the length used to average the signal from the scanning electron microscope and to obtain the position of a single edge point. Good results for this so called summing length were obtained for values above 100 nm. The second important parameter is the total length of the investigated line that is used to calculate the LER. Here, we found that the increase of LER values with increasing investigation length is similar to the well-established behavior on wafers. It was found that the average LER value calculated from various sites saturates at investigation lengths larger than 10 µm, whereas single LER results show no scattering within measurement precision for investigation lengths larger than 30 µm independently of mask position. In comparison to similar investigation on wafer both the summing length as well as the investigation length have to be chosen about one magnitude larger. It is suggested that the multi exposure process of mask creates roughness on length scales of the order of several micrometers.