The gas-in-oil concentrations are used to diagnose oil-filled transformer insulation. According to National Criteria, the measuring period for gas-in-oil concentrations of oil-filled transformer is 3 months at least, and besides, the measuring procedure is complicated. As on-line measurements of gas chromatography are not completely implemented, it is difficult to acquire the gas-in-oil concentrations timely. For making up the disadvantages, a new method based on Grey Theory to predict the gas-in-oil concentrations is proposed in this paper. Using this method, the gas-in-oil concentrations at any future time can be predicted, hence the insulation of oil-filled transformer can be also diagnosed. It is approved to be a simple, accurate and practically useful method to predict the gas-in-oil concentrations of oil-filled transformer and other analogous electrical equipment.