Hydraulic fluids for automotive chassis systems are expected to have excellent low-temperature fluidity to get quick response at low temperature. A synthetic active suspension fluid has been designed by analyzing viscosity-temperature characteristics of Poly Methacrylate (PMA) / Poly a -Olefin (PAO) or diester mixtures based on the theoretical formula proposed by Carrasco et al.Measured kinematic viscosities are correlated with calculated ones very well at high temperature. On the other hand, there is some difference between measured and calculated low-temperature viscosities due to wax crystallization resulting from the mineral oils used as carrier oil of polymers. The theoretical formula of Carrasco et al. is applicable to polymer / synthetic base fluid mixtures even under the arctic condition in the case of no wax crystallization. PAO has better wax solubility and low-temperature fluidity than diesters judging from the fact that there is less difference between measured and calculated low-temperature viscosities of PAO than that of diesters.A synthetic base fluid consisting of a mixture of PAO and diester at the ratio of 80 to 20 was adopted to develop an active suspension fluid, considering its low-temperature fluidity and material compatibility.