Recently, researchers have become interested in natural ester fluids, as they are an environmentally friendly alternative to mineral oils. Natural ester fluids are a natural resource made from plants; they have higher biodegradability, flash, and fire points, and a greater permittivity compared to conventional mineral oils. However, natural ester fluids also have a higher pour point, viscosity, and water content. These characteristics can hamper circulation and impair the electrical properties of an oil-filled transformer. A large amount of data has been accumulated over the years in regards to mineral insulating oil involving dielectric breakdown voltage and lightning impulse tests. However, natural ester fluids have not had their electrical properties sufficiently characterized. In this paper, we present an investigation into the characteristics of the electrical discharge development in natural ester fluids and in an oil-filled transformer near the pour points. The experiment results show that the electrical properties decreased according to a decrease in the ambient temperature and freezing time. It was found that the pour point and water content of natural ester fluids have a significant effect on the electrical properties.