A computer aided design system comprises an interface creating module, a selecting module, a filling module and a measuring module. The interface module creates a user interface to display the design on the screen of the device with a plurality of to-be-checked patterns. The selecting module selects a pattern. The interface module further creates a parameter setting interface. The parameters comprises a predetermined width, the space value between the adjacent parallel filled lines and the rotation degree of the selected pattern. The filling module draws filled lines in the selected pattern according to the parameters. The measuring module detects whether the length of each filled line is at least the predetermined width value, if the length of the filled line is less than the predetermined width value, the dimension of the selected pattern is unqualified and the measuring module highlights the filled lines.