A bidirectional thrust bearing assembly is used between a downhole fluid motor and a rock bit for drilling oil wells. The bearing assembly has a stationary housing with radial journal bearing sleeves and a rotatable drive shaft also having radial bearing sleeves. A pair of oppositely facing thrust bearing rings are mounted in the housing. A second pair of thrust bearing rings are mounted on the shaft so as to have faces opposing the bearing faces on the first pair of rings. Belleville springs resiliently bias a pair of the thrust bearing rings apart and carry the thrust load between such rings. Each ring has a plurality of inserts of hard material, preferably polycrystalline diamond, at the bearing surface. Means are provided for circulating drilling fluid from the motor through the thrust bearing faces for forming hydrodynamic fluid bearing films in the bearing interfaces.