The lifespan of the oil-immersed transformers is predominantly controlled by deterioration of the insulation paper surrounding coils. The deterioration of the insulation paper is caused by the degradation of cellulose polymer due to the heat generated at the coils. The decrease of mechanical strength of the insulation paper caused by the reduction in the average degree of polymerization of the insulation paper makes it impossible to follow the deformation of the coils during operation. The present paper proposes a way to estimate the lifespan of the insulation paper based on the rate of depolymerization of cellulose polymer. The estimation takes into consideration the effect of temperature rise due to the heat generation in the transformer. The proposed method of identifying the residual lifespan of the oil-immersed transformers has proved to be valuable in devising an efficient strategy of maintenance.