We have synthesized $CaS:Eu^{2+}$ phosphor by using a sealing vessel and evaluated its photoluminescence properties. The method using a sealing vessel is simple and economical in comparison with other methods reported up to date. As an activator concentration was increased from 0.1 mol% to 4 mol%, the main emission wavelength of the phosphor was increased from 642 nm to 651 nm due to the crystal field splitting of 5d levels of $Eu^{2+}$ ion. Although the same amount of $Eu_2O_3$ was used, the concentration of the activator ions, which were reduced from $Eu^{3+};to;Eu^{2+}$ and substituted $Ca^{2+}$ ions, was increased with increase of firing temperature. Therefore, the main emission wavelength was also. shifted from 645 nm to 651 nm with increase of firing temperature from $1100^{circ}C;to;1300^{circ}C$. The critical distance($R_{c1}=20.65;{AA}$) between $Eu^{2+}$ ions calculated from the critical concentration was well matched with that($R_{c2}=19.17;{AA}$) calculated from the Dexter formula.