Controlling line width error is one of the most challenging tasks in mask production. Current methods for monitoring line width are based on either CD measurements or edge defect detection. Neither method guarantees the detection of line width errors with good sensitivity over the whole mask area. For example, a combination of small edge defect together with CD non-uniformity within the allowed tolerance, can yield a significant line width error, which will not be detected using conventional methods. In this paper we are presenting a new approach for line width error detection. The method, a new feature of Orbot's RT-8000 die-to-database Reticle Inspection System, is a sensitive detector that operates during the inspection of all the reticle's area. The detection is based on a comparison of measured line widths on both the database and the scanned image ofthe reticle. In section 2, the motivation for this new detector is driven. An analysis of various defect types, which are difficult to detected using edge detection approach or CD measurements, is presented. In section 3, the basic concept of the new approach is introduced together with a description of the new detector and it's characteristics. In section 4, the calibration process that took place in order to evaluate the sensitivity of the detector is described. The experimental results of this evaluation are reported in section 5.