Surface reaction mechanism of Tri-Methyl-Gallium (TMG) on GaAs is investigated by using the quadrupole mass spectroscopy (QMS). The present QMS measurements, which is devoted only to the surface reaction, distinguish the surface reaction of TMG on {100} GaAs into two sorts. By taking into account with the QMS results of {111}Ga and {111}As surface, the surface reaction mechanism on GaAs is discussed. These results are available to the molecular layer epitaxy (MLE) of GaAs with high purity.