Model-based library (MBL) method, based on the fundamental physics of electron-solid interaction and Monte Carlo simulation of electron transport and secondary electron (SE) cascades, is an ideal algorithm in critical dimension (CD) metrology by CD-SEM, which exceeds the experiential structure characterization with SE profile curve. Particularly, comparing to the threshold method, the MBL method can extends to 3D metrology of CD by including many more geometrical structural parameters, such as, top CD, bottom CD, height and sidewall angles. In this work, we study the SE signal profile shape in CD metrology for the MBL method. The gate lines in trapezoidal cross section shape with arc corners are modeled for the simulation of SE linescans. The sensitivity of the SE intensity profiles to the different topographic parameters are investigated. In addition, the practical aspects of CD evaluation including pre-processing of experimental SE image and profile matching procedure are discussed. The work will be beneficial to the related standardization of CD measurement.