The surface kinetics of MBE growth of (100) Ga0.5 Al0.5As are studied theoretically using a stochastic model which is based on master equation with random distribution approximation. The kinetic processes considered are: adsorption and inter- and intra-layer migrations for Ga, Al and As. Both monoatomic and diatomic As molecular species are considered. The model parameters such as the atomic pair interaction energies, migrational requency and activation energy were obtained from available experimental and theoretical data. The surface ordering kinetics are studied as a function of the fluxes (2Å/sec. for cations), flux ratio (1 : 10, 1 : 20 and 1 : 30 for cation to anion) and substrate temperature (775° – 923°K). Degree of ordering was obtained in terms of a short range order (SRO) parameter. The order-disorder temperature defined as the temperature of maximum order (above and below which the order parameter decreases) was obtained for flux ratios of 1 : 10 and 1 : 20. Both As and As2 species were considered. The order-disorder temperature was found to increase as the flux ratio was changed from 1 : 10 to 1 : 30, while the maximum degree of ordering decreased slightly. The results are in good qualitative agreement with experiments. The surface ordering kinetics observed can be described in terms of effective surface migration rates of the cations as follows. Lower temperature and higher flux ratio yield smaller effective surface migration rates for cation, which results in lesser degree of ordering. Beyond the transition temperature, the thermal energy is large enough to thermally randomize the atom pair bonds, thereby decreasing the degree of ordering. The higher transition temperature for higher flux ratios is attributed to lower effective migration rate.