In this paper, we have studied the dependence of insertion loss of a microstrip stepped impedance lowpass filter with coupled open stubs. Coupling mechanisms in the filter depend not only on the transmission line width of the filter but also the gap width of coupled open stubs and three attenuation poles are created with the proper conditions. Also edge capacitance between open stubs play an important role in having three attenuation Poles. We verify the results by obtaining [S] matrices.