In this article, the quasi-TEM approach is successfully used to analyze coupled-microstrip lines of finite length, which are usually used for the coupling between the source/load and resonators in microstrip filters. First, an orthogonal expansion method is used to extract the per-length inductance and capacitance matrices. The authors then obtain the impedance matrix or admittance matrix, which relates the voltages and currents at both ends of coupled-microstrip lines. To verify our theory, an example is presented, i.e., a Y-shaped dual-mode bandpass microstrip filter, for which, the equivalent circuit model based on the quasi-TEM analysis of coupled-microstrip lines is accurately set up. The approach in this article is applicable to planar microstrip lines with arbitrary number of conductors, under the quasi-TEM assumption. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2012.