This document specifies the structure model with related parameters, file format and fitting procedure for characterizing critical dimension (CD) values for wafer and photomask by imaging with a critical dimension scanning electron microscope (CD-SEM) by the model-based library (MBL) method. The method is applicable to linewidth determination for specimen, such as, gate on wafer, photomask, single isolated or dense line feature pattern down to size of 10nm.