To elucidate the properties of binary system of room temperature ionic liquids (RTILs), we measured and compared the temperature dependence of densities, viscosities, and refractive indices of seven binary RTILs as well as neat RTILs. The observed densities, viscosities, and refractive indices of the binary systems showed intermediate properties between those of two starting RTILs. The values observed for the binary RTILs were only deviated from the values estimated based on the data of neat RTILs (+/- 0.5%), and this deviation depended on both the combination of RTILs and the structures of the cation/anion. These results indicate that the properties of RTILs can be easily tuned by mixing RTILs depending on their applications.