In this paper, we establish the existence of solitary wave in some KdV-mKdV equation with dissipative perturbation by applying geometric singular perturbation technique and Melnikov function. The distance of the stable manifold and unstable manifold are computed to show the existence of the homoclinic loop for the related ordinary differential equation systems on the slow manifold, which implies the existence of a solitary wave for the KdVmKdV equation with dissipative perturbation