Summary: We develop a posteriori error estimation of mixed discontinuous Galerkin finite element approximations of Stokes problem. In particular, we derive computable upper bounds on the error, measured in terms of a natural (mesh-dependent) energy norm. This is done by rewriting the underlying method in a non-consistent form using appropriate lifting operators, and by employing a decomposition result for discontinuous spaces. Finally, we present a series of numerical experiments highlighting the performance of the proposed a posteriori error estimator on adaptively refined meshes.