Abstract: For crests and troughs exiting in the scattering parameters of the coupled microstrip lines, the relation between the frequencies where crests and troughs occur and the parallel length, space between lines for the four-port network is deduced from scattering parameters of the odd mode and even mode based on microwave network principle. Effect on the transmitted signal by the insertion loss for the structure of two parallel microstrip lines is also studied in frequency domain and time domain respectively. Due to the coupling between microstrip lines, it is shown that the frequencies where crests and troughs occur increase with the space between lines but reduce with the parallel length, and the transmitted signal is attenuated when crosstalk is induced.