Influence of preparation conditions on the electrical conductivity, dissipation factor, and hardness of silicone rubber vulcanized by the condensation reaction using dibutyltin dilaurate as catalyst was studied. Thickness of rubber specimens and amounts of catalyst added were varied. Electrical properties were measured at 20–200°C for rubber specimens heat-treated at 200°C for 10 h under various conditions, for example, heat treated in air, but in a sealed state in order to inhibit volatilizaions of the catalyst and vaporization of by-products, or heat treated in a dry nitrogen atmosphere, in order to avoid oxidation and hydrolysis of the catalyst. Furthermore, the catalysts and byproducts were volatilized in air from both surfaces of the specimen in order to accelerate volatilization. Degradation of silicon rubber with heat treatment was elucidated by hardness measurements and hardness dependence of the electrical conductivity of the specimens was analyzed.