The variation of electrical parameters during microarc oxidation (MAO) of titanium alloys at constant voltage has been studied by means of a laboratory-made data acquisition system. Experimental results show that the variation of the cathodic and anodic current amplitudes and effective working current in an electrical loop with the duration of treatment show obviously four stages, and the variation of the coating thickness with the treating time has obviously three stages, and the variations of dynamic forward and backward resistance as well as resistance rate of coatings also show different stages. During MAO process, the dynamic forward resistance is always different from the backward resistance, and the former is generally greater than the latter. Scanning electron microscopy shows that the surface morphology of MAO coatings is porous and varies with treatment time in steps.