It is studied how to reach the resolution and alignment limit of a mask aligner with PLA 501F as a instance in this paper.It is stressed that the mutual coordination of light source,photo resist,filter and main lens,is the key point to reach the resolution limit of a photo lithographer,the alignment pattern design and the alignment skill play an in portent role in improving the alignment accuracy of a photo lithographer.Moreover the merits and de merits of contact and proximity exposure is analyzed in the paper.