Suppose that d ≥ 1 and 0 < β < α < 2. We establish the existence and uniqueness of the fundamental solution q b (t, x, y) to a class of (typically nonsymmetric) non-local operatorsis a strictly positive continuous function and it uniquely determines a conservative Feller process X b , which has strong Feller property. The Feller process X b is the unique solution to the martingale problem of (L b , S(R d )), where S(R d ) denotes the space of tempered functions on R d . Furthermore, sharp two-sided estimates on q b (t, x, y) are derived. In stark contrast with the gradient perturbations, these estimates exhibit different behaviors for different types of b(x, z). The model considered in this paper contains the following as a special case. Let Y and Z be (rotationally) symmetric α-stable process and symmetric β-stable processes on R d , respectively, that are independent to each other. Solution to stochastic differential equations dX t = dY t + c(X t− )dZ t has infinitesimal generator L b with b(x, z) = |c(x)| β .