  We study the dynamics of charm quarks in the partonic medium and its
implication to the di-electron spectra in high energy heavy ion collisions. The
charm quarks traversing a thermal medium is simulated by the relativistic
Langevin equation for elastic scatterings of charm quarks by thermal partons in
an expanding fireball. The transport coefficients of charm quarks are
calculated by the in-medium T-matrix method, where a static heavy quark
potential is used with parameters fitted by the lattice QCD results. The
di-electron invariant mass spectra are computed in most central collisions and
are compared to the STAR data. The angular correlations of di-electrons are
almost the same in $p+p$ and Au+Au collisions in the mass range
$1.1<M<2.5~\mathrm{GeV/c^2}$ with the back-to-back feature. This means that the
angular correlation is intact even with medium interaction at the RHIC energy.
