For viscous couplings(VCs) as a driving force transmission system of vehicles, requirement of torque characteristics has been getting very stringent. Because the torque characteristics significantly affect four wheel drive vehicles' abilities such as traction performance and driving stability. Furthermore, the recent concerns on high fuel economy, low pollution and low cost require that design of VCs should be increasingly compact, light weighted and excellent in transmitted torque's stability.It is an easy way to increase viscosity of viscous coupling fluids(VCFs) for the compact design of the VC. But it might cause increase in heat load and wear of plates which resulted in degradation of the VCF. The degradation affects VCF's viscosity and impairs stability in torque transmission. Therefore it is indispensable to develop high viscosity VCF which is excellent in long-term viscosity's stability. The author investigated the factors of viscosity increase and the new prescription of additives which can satisfy the requirements described above.As a result, by means of combining several additives, the VCF of high viscosity(20,000mm2/s increased from previous fluid's 7,000mm2/s) and long-term stability. The additives consists of several types of anti-wear agents, one of which is effective in suppressing the wear in an early stage, and the other contributes to the long-term stability.The new VC with this newly developed VCF achieved 25% of weight reduction, leading to cost saving, while keeping equivalent durability to the previous VC.In addition, it is confirmed that the rigidity and the slit shape of the plate, another important part of VC, had a large effect on the degradation of VCFs.