The law of wet oxidation rate is studied with experiments in vertical-cavity surface-emitting lasers (VCSEL) in order to accurately control the oxidation aperture. Wet oxidation experiments are carried out upon the samples of VCSELs at different temperatures, and micro-probe analyses are made at different oxidation depth by scanning electron microscope (SEM). It shows that the element content in oxidation products is different at different oxidation depth especially that of the oxygen element. The reaction style and the oxidation products are analyzed and discussed, and a mathematical model on oxidation rate vs time is built up. The oxidation rate is derived, which is in an exponential growth law during the wet oxidation. At constant temperature, the oxidation rate becomes more and more steady with increase the oxidation time. The oxidation process quality can be improved by lowering the oxidation temperature and prolonging the oxidation time appropriately.