Polylactic acid (PLA) is a bioplastic and a biomass material that is made from corn starch, etc. Having studied the potential of PLA as an electrical insulation material, the authors have demonstrated that PLA has high resistivity, high dielectric breakdown strength and other outstanding electrical insulation properties equivalent to those of low-density polyethylene (LDPE) and other petroleum-based plastics within a temperature range from room temperature to approximately 70°C. The degree of crystallinity of PLA was increased by heat treatment, and the dielectric breakdown strength (E_B) and mechanical properties of the obtained PLA of different spherulite sizes were studied. PLA heat-treated at a temperature of 120°C or higher showed rapid increases in spherulite size. At both measurement temperatures of 25°C and 80°C, the E_B showed rapid falls when the heat treatment temperature was 120°C or higher. At a measurement temperature of 25°C, the tensile strength at break and breaking elongation showed rapid falls when the heat treatment temperature was 120°C or higher.