HgS and HgS: Co crystals and films grown by the slow cooling and the chemical bath deposition method were used to measure their crystal structure and their optical absorption spectra. HgS and HgS: Co crystals are hexagonal structure with the lattice constant $a_0=4.155{AA}$, $c_0=9.505{AA}$ for HgS and $a_0=4.148{AA}$, $c_0=9.462{AA}$ for HgS and $a_0=4.135{AA}$, $c_0=9.442{AA}$ for HgS: Co, respectively. The optical energy gap of these crystals are given as 2.040 eV for HgS and 1.900 eV for HgS: Co, and the optical energy gap of these films were 2.440 eV for HgS and 1.940 eV for HgS: Co at room temperature, respectively.