Optical proximity correction (OPC) is an essential technology for critical dimension (CD) control in Low-k1 lithography. As technology node becomes tighter, more aggressive OPC is required. However, the number of so-called HOT-SPOTS is increasing dramatically. To apply OPC correctly and efficiently, we should consider the total optimization of the process in close connection with data processing, reticle and wafer fabrication process. Conventional one-dimensional CD measurement is no longer suitable for complicated two-dimensional (2D) patterns generated by OPC (e.g. JOG and SERIF). For quality assurance of mask pattern, a metrology of complicated 2D OPC patterns has been required. In our previous report, we proposed a lithography simulation based on edge extraction from a fine pixel SEM image of an actual photomask. This method is very effective for evaluating quality of 2D OPC mask patterns. Employing the method, we developed a system for guaranteeing 2D OPC patterns before shipping the mask to a wafer factory (Fig. 1). In PMJ2005, we presented some specifications required for an SEM, which was one of the key factors of this method. We estimated how factors such as field of image, image resolution, positioning error, and image magnification affect lithography simulation based on fine pixel SEM image. For mask pattern quality assurance of hp65, we found that the field of image of larger than 16μm square, the pixel size of less than 3nm, the positioning error of within +/- 1μm and the magnification error of less than 0.3% were acceptable (Table 1). Under these conditions, wafer image can be predicted with sufficient accuracy by the simulation. And then, in BACUS2005, we reported on a new SEM that was able to satisfy these specifications. In this paper, we report some evaluation results of distortion caused by not only magnification error but also rotation and position error using actual fine pixel SEM image. We will also present our evaluation results of the errors in various pattern conditions such as Dark Field/Bright Field, Pattern density.