Cobalt doped GaAs thin films with the thickness of 100 nm and 200 nm were deposited on glass substrates by a thermionic vacuum arc system. The optical and surface properties were investigated as functions of the film thickness. The refractive index of the films decreased with the increasing thickness. A band gap value of 1.42 eV was obtained for the 100 nm film and 1.38 eV for the 200 nm film from the Tauc plots of (alpha hv)(2) vs. h.. According to the results obtained from the AFM studies, the root mean square surface roughness of the films were 1.52 nm and 3.13 nm for the 100 nm and 200 nm film, respectively.