A viscous fluid coupling includes a housing rotatably supported to a drive shaft of an engine. An operation plate set in the housing divides a space of the housing into a reservoir and an operation chamber. The operation plate has a communication hole communicating the reservoir and the operation chamber. A rotor fixed to the drive shaft is disposed in the operation chamber. A valve mechanism installed to the operation plate to close and open the communication hole according to ambient temperature of the housing. A driven wheel fixed to the housing is located between the operation plate and the rotor. A torque transmitting section includes a first annular projection concentrically formed on the driven wheel and a plurality of second annular projections concentrically formed on the rotor. The first annular projections are overlappedly adjacent to the second annular projections so as to establish fluid coupling therebetween through the viscous fluid.