This paper presents a selfconsistent IV simulation technique for an RTD with defect wells placed inside the barriers. The motivation of this paper was to model the excess valley current by a defectassisted tunneling mechanism. We have calculated the transmission coefficients and IV characteristics self consistently with Poisson's equation coupled to quantum mechanical tunneling through the barrier. The shape of transmission coefficient was broadened and greatly enhanced in the offresonance region when the defect well was introduced in the barriers. Our results gave a good qualitative estimation of the valley current and the peak to valley current ratio (PVCR).