This paper defines an effective microwave surface resistance Reffs for the nonuniform distribution of microwave surface resistance Rs in the strip of a microstrip. It is proved that Reffs is equivalent to the expression of Rs used in experiments, and that the Reffs is dominated by the edge part, i.e., the area of width λ2/2t from the strip edge, where λ is the magnetic penetration depth and t is the film thickness. Under the assumption that Rs ∼ (Hyf)n where Hyrf is the component of rf magnetic field along the film thickness and n is an integer, the ratio of the contributions of the edge part and the rest of the strip to Reffs is calculated by using an approximate analytical expression of the surface current density distribution Js in the strip and Hyrf calculated by the London equation. The effect of film's edge on Rs was studied using a microstrip resonator. It is found that the perfectness of the edge could affect the magnitude of the power dependence of Rs significantly, which agreed with our analysis.