The variation of electrical parameters during microarc oxidation(MAO) of aluminium alloys at constant voltage has been studied by means of a laboratory-made data acquisition system. Experimental results show that the variations of the cath odic and anodic current amplitude and effective working current in an electrical loop with the duration of treatment show obviously five stages, and the variati ons of dynamic positive resistance and the rate of change of resistance in coati ngs also show different stages, while the variations of dynamic negative resista nce and the rate of change of resistance with oxidation time are not evident. Du ring the MAO process, the dynamic positive resistance is always not equal to the negative resistance, and the former is generally greater than the latter in gen eral. Scanning electron microscopy analysis shows that the surface morphology of MAO coatings is macro-porous and its variations with treatment time are in ste ps.