Novel analytical method based on extended spectral domain approach (ESDA) is presented for cylindrical microstrip line. The method utilizes the aperture fields as the source quantities, as opposed to the conventional methods, which have used the current on the strip as the source. The whole region can be divided into sub-regions by the introduction of aperture fields, and each sub-region can be treated independently. This method makes possible the analysis both of zero and finite thickness of the strip conductor. The numerical procedure incorporates the effects of the edge singularities properly and can afford the efficient and accurate calculations for the phase constants and characteristic impedances of a microstrip line with zero- and finite-thickness conductor. The calculated results by the present method reveal the effect of conductor thickness on the characteristics of a cylindrical microstrip line.