The mechanical and dielectric breakdown properties of polylactic acid (PLA) and PET copolymer, which are biodegradable plastics, were examined, and physicochemical analysis were performed. At room temperature, the Young's modulus of PLA and PET copolymer were about 3.6 GPa and about 3.9 GPa, respectively. Moreover, the temperature to which Young's modulus of PLA and PET copolymer begins to decrease was about 60°C and about 55°C, respectively. The dielectric breakdown strength (E_B) of PLA and PET copolymer at room temperature were about 6.2 MV/cm and about 4.8 MV/cm, respectively. The E_B of PLA increased with an increase in temperature, and was about 6.9 MV/cm at 60°C (∂E_B/∂T≥0). The E_B of PET copolymer was hardly dependent on temperature in the domain of 70°C or less (∂E_B/∂T≅0). In contrast with it, when it became a high temperature region, EB of PLA and PET copolymer decreased with the rise of temperature (∂E_B/∂T<0)