A periodic surface integral formulation is proposed to analyze the reflection and transmission properties of a lossy periodic composite structure which has circular conducting fibers embedded in a dielectric matrix. This formulation is based on the equivalence principle which represents the unknown electric and magnetic currents over the material discontinuity interfaces, and uses the structure periodicity and Poisson summation formula to reduce the problem to a periodic cell. These surface integral equations are then solved numerically, using the method of moments with pulse bases and point matching. Only the transverse magnetic (TM) case is analyzed and the numerical results such as reflected, transmitted, and dissipated powers for a single-layer fiber-reinforced composite structure are presented, in detail, to discuss the effects of frequency, incident angle, fiber radius, fiber conductivity, embedding dielectric, etc. A convergence study and a comparison with the previous published results are also included to confirm the accuracy of the new formulation.