  We will report on the first full calculation of the $K_L-K_S$ mass difference
in lattice QCD. The calculation is performed on a 2+1 flavor, domain wall
fermion, $24^3\times 64$ ensemble with a 329 MeV pion mass and a 575 MeV kaon
mass. Both double penguin diagrams and disconnected diagrams are included in
this calculation. The calculation is made finite through the GIM mechanism by
introducing a 949 MeV valence charm quark. While the double penguin diagrams
contribute a very small fraction to the mass difference, there is a large
cancellation between disconnected diagrams and other types of diagrams. We
obtain the mass difference $\Delta M_K$=$3.30(34)\times 10^{-12}$ MeV for these
unphysical kinematics.
