The advanced lithography needs to be tightly controlled in various areas of lithography. The mask CD specification is one of new areas required much tighter control. Typically, mask CD error can be sorted as two different categories. One is Mean-to-target (MTT) and another is CD uniformity (CDU). The MTT is the difference between the target value and the average value of the measured CD on the mask. CDU means CD uniformity across mask. Those two potential errors can be magnified on the wafer level due to the MEEF. To overcome the MTT, we can adjust expose dose to compensate mask CD error so that we achieve targeted CD on the wafer level. However, the changing expose dose also induces process window change due to the MEEF. It means that we have narrower process window even if we get the targeted CD on the wafer level. On the other hand, CDU can give two different effects on the wafer level. One is narrower process window due to magnified ACLV (Across Chip Line-width Variation) due to the MEEF. Another effect of CDU is the poor OPC accuracy caused by different MEEF as function of pitch. For example, we assume that CD difference of dense line and isolated line is 10 nm on the mask. However, on the wafer, this 10 nm can be magnified as 20 nm by MEEF difference between two structures. Therefore, we think that the mask specification needs to take account those effects. In this paper, we will show technical data to prove how MTT and CDU impact on process window and OPC accuracy. And we will show how we have to make mask specification to overcome those effects.