A direct current compensation method for S-parameter rational functions reads S-parameters f(sk), which are generated at given frequency sk, from a storage unit. An S-parameter, which is generated at frequency sk=0 is supplemented into the S-parameters f(sk), upon the condition that there is no S-parameter which is generated at the frequency sk=0. An S-parameter ration function is generated according to the S-parameters f(sk). A DC level of the S-parameter rational function is compensated to generate a compensated S-parameter rational function.