In this thesis the lattice parameters and the optical properties of $Cd_{1-x}Co_xSe$ were studied. The lattice parameters of wurtzite CdSe were a=4.297 Å and c=7.008 Å by Cohen's method through X-ray diffraction experiment. As x increasd, the lattice parameters decreased very slightly and the lattice structure was not changed. The absorption spectrum showed $Co^{2+}$ energy levels. $E_2$ transiton energy was 0.720 eV and $E_3$ transition energy, which was splitted into 3 peaks by the spin-orbit interaction, was 1.549 eV. Racah parameter B and crystal field parameter $D_q$ were $544cm^{-1}$ and $338cm^{-1}$ respectively. From the reflection spectrum of CdSe the band gap energy $E_g$ was 1.73 eV.
