  We consider the complement value problem for a class of second order elliptic
integro-differential operators. Let $D$ be a bounded Lipschitz domain of
$\mathbb{R}^d$. Under mild conditions, we show that there exists a unique
bounded continuous weak solution to the following equation $$
\left\{\begin{array}{l}(\Delta+a^{\alpha}\Delta^{\alpha/2}+b\cdot\nabla+c+{\rm
div} \hat{b})u+f=0\ \ {\rm in}\ D,\\ u=g\ \ {\rm on}\ D^c. \end{array}\right.$$
  Moreover, we give an explicit probabilistic representation of the solution.
The recently developed stochastic calculus for Markov processes associated with
semi-Dirichlet forms and heat kernel estimates play important roles in our
approach.
