The invention is directed to a mark pattern for forming a process monitor mark in a patterned underlayer to monitor a patterning result of a photoresist layer over the patterned underlayer around the boundary between a peripheral region and a device region of a die, wherein the patterned underlayer is formed by using a first mask having a first pattern in a main region of the first mask and the mark pattern at an unused region of the first mask and the first pattern possesses a first mask critical dimension. The mark pattern comprising: a second pattern and a frame pattern. The second pattern has a second mask critical dimension, wherein the second mask critical dimension is as same as the first mask critical dimension. The frame pattern encloses the second pattern.