The quality of GaAs epitaxial layers prepared by molecular layer epitaxy (MLE) with and without UV light irradiation was studied and appears to be a promising method for preparing the GaAs thin layers with an atomic order accuracy. Namely, single layer-by-layer growth has been realized for GaAs using AsH3 and trimethyl (TMG), and triethyl (TEG), gallium. Substrate temperature of 500 °C for TMG and AsH3, and 300 °C for TEG and AsH3 fulfilled the conditions for monolayer growth, where the film growth thickness per cycle corresponding to 2.83 and 3.26 Å for (100) and (111) B faces, respectively, are nearly saturated independently of the gas admittance quantity. Influences of photoirradiation on the film growth have been investigated by use of excimer lasers and Hg lamps. UV light irradiation improved the surface morphology and in certain instances also improved impurity concentrations. Conditioned photoirradiation—light beam synchronized with the time duration of AsH3 admittance, MO-gas admittance, and evacuation of gaseous molecules—was studied.