CDU is probably one of the most important process control parameters for poly gate printing process. As the design rule has been shrunk to below 100nm node and k1 approaching 0.35, the required specifications for CDU are more stringently tightened. For low k1 lithogrpahy, optical proximity effect can severely impact CDU of the cirtical features from different neighboring environments, such as, isolated versus dense regions, or near the crowded corners and intersections, etc. Conventional model OPC has been assumed to correct mask feature CD at a best process setting condition. The approach has been widely adopted but it is increasingly becoming less satisfactory since this method does not take into account the process variation within the acceptable process window. In ASMLs LithoCruiser, for a selected CD cut-line, we use a Monte Carlo approach to randomly generate process conditions within the process window to simulate thousands of CDs. Based on the predicted CD distribution, we can calculate the predicted CDU Yield within the process window. Using LithoCruiser's Application Programming Interface (API) we can set up an optimization loop to adjust mask feature snippets for a desired imaging and process condition to achieve user specified CDU yield. Depending on the neighboring environment, mask snippets with the same target CD may need different biases and so are the features with different target CDs due to different degrees of optical proximity effects.