In recent years, environmental concerns have been raised on the use of poorly biodegradable fluids in electrical apparatus in regions where spills from leaks and equipment failure could contaminate the surroundings. The newly invented vegetable insulating oil is highly biodegradable and have negligible impact on the environment, human health and the ecosystem. For development of the environmental-friendly pole transformer using vegetable insulating oil, the dielectric constructions of the pole transformer were discussed in this paper. Depending on the dielectric constructions, the AC breakdown characteristics of the Nomex insulating papers and the vegetable insulating oil were studied by simulated electrode systems. Based on the experimental results, the maximum design stress($E_{max}$) for insulation design of the environmental-friendly pole transformer were suggested.