Mo thin films were used for the back electrode because of the low resistivity in the Mo/$CuInGaSe_2$ contact in chalcopyrite solar cells. $1;{mu}m$ thick Mo thin films were deposited on soda lime glass by varying the Ar pressure with the dc-magnetron sputtering process. The effects of the Ar pressure on the morphology of the Mo back electrode were studied and the relationships between the morphology and electro-optical properties, namely, the resistivity as well as the reflectance of the Mo thin films, were investigated. The resitivity increased from $24;{mu}{Omega}{cdot}cm$ to $11833;{mu}{Omega}{cdot}cm$; this was caused by the increased surface defect and low crystallinity as the Ar pressure increased from $3{ imes}10^{-3}$ to $3{ imes}10^{-2};Torr$. The surface morphologies of the Mo thin films changed from somewhat coarse fibrous structures to irregular and fine celled structures with increased surface cracks along the cell boundaries, as the Ar pressure increased from $3{ imes}10^{-3}$ to $3{ imes}10^{-2};Torr$. The changes of reflectances in the visible light range with Ar pressures were mainly attributed to the surface morphological changes of the Mo thin films. The reflectance in the visible light range showed the highest value of 45% at $3{ imes}10^{-3};Torr$ and decreased to 18.5% at $3{ imes}10^{-2};Torr$.