As the IC design rule is getting smaller, mask writer's accuracy plays a more important role in mask making. In order to reduce writer's butting error, high end writers all use multi-pass writing technique. Theoretically, the more passes are used, the smaller butting error is expected. When writer with multi-pass technique has encountered abnormal sectors in the pattern loading system, extra or missing bits might cause CD variation on the paste. We found high end masks with CD variation smaller than 0.15 micrometers written by an eight pass writing system. It is very important to be able to detect this kind of CD error before the mask is shipped. In this paper, we will discuss TMC's approach to the detection of CD error caused by the writer.