This paper proposes a multi-coefficient estimation method for the dielectric parameters of rough materials and an effect level-based parameterization method for diffuse scattering models to characterize and model the diffuse scattering propagation at millimeter-wave frequencies. A series of diffuse scattering propagation measurements and simulations for rough materials have been performed at 40–50 GHz in a typical indoor scenario. Theoretical reflection coefficient, transmission coefficient, and scattering coefficient of rough materials, which are requisite for the proposed estimation method, are derived based on the Fresnel theoretical model and the Gaussian rough surface model. The directive model and double-lobe model are chosen and integrated with ray tracing tool to simulate the diffuse multipath propagation for rough materials based on effect level evaluation results. The optimal model parameters are obtained and various simulation results are compared and in particular, the estimated ranges of scattering coefficients agree well with the measured values. The investigations demonstrate that the proposed parameterization method is reliable and accurate for the diffuse scattering models and can be applied for the determination of model parameters from extensive materials measurement data, especially for millimeter-wave channel analysis and modeling.