::title::A Priori Error Estimates of a Combined Mixed Finite Element and Discontinuous Galerkin Method for Compressible Miscrible Displacement with Molecular Diffusion and Dispersion ::author::1::Jiming Yang ::author::2::Yanping Chen ::address::1::College of Science, Hunan Institute of Engineering, Xiangtan 411104, China ::address::2::School of Mathematical Sciences, South China Normal University, Guangzhou 510631, China ::email::yangjiminghnie@163.com, ypchen@xtu.edu.cn, yanpingchen@scnu.edu.cn ::receive::Received 2008-12-6 Accepted 2010-2-4 ::online::Available online 2010-9-20 ::DOI::10.4208/jcm.1006-m2991 ::keywords::A priori error, Mixed finite element, Discontinuous Galerkin, Compressible miscible displacement. ::ams::65M12, 65M60. ::abstract:: A combined approximation for a kind of compressible miscible displacement problems including molecular diffusion and dispersion in porous media is studied. Mixed finite element method is applied to the flow equation, and the transport one is solved by the symmetric interior penalty discontinuous Galerkin method (SIPG). To avoid the inconvenience of the cut-off operator in \cite{p1,p15}, some induction hypotheses different from the ones in \cite{p18} are used. Based on interpolation projection properties, a priori $hp$ error estimates are obtained. Comparing with the existing error analysis that only deals with the diffusion case, the current work is more complicated and more significant.