Charm quarks are produced in the early stage of high-energy collisions and thus provide a unique tool to probe the partonic matter created in relativistic heavy-ion collisions at RHIC energies. The direct measurement of $D$ mesons with low $p_T$ coverage allows to extract important information on the scaling properties of the charm production cross-section and their transport properties in the medium. We will report the first results of directly reconstructed $D$ mesons via their hadronic decay products in Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV up to $p_T\sim$3 GeV/c. Single electron spectra from the charm semi-leptonic decays are also analyzed from the same data set using the STAR TPC, TOF and BEMC detectors. The total cross-sections and the nuclear modification factor $R_{AA}$ of open charm hadrons are presented. The spectra from Au+Au collisions are compared to that in d+Au collisions [PRL 94 (2005) 062301] to infer information of the level of radial flow of charmed mesons.