A hydraulic transmission system comprising a housing having at least an oil tank containing oil, an oil pump mounted in the housing and having an inlet port and an outlet port, an oil motor mounted in the housing and having an inlet port and an outlet port, a high-pressure oil passage defined in the housing and interconnecting the outlet port of the oil pump and the inlet port of the oil motor, a low-pressure oil passage defined in the housing and interconnecting the outlet port of the oil motor and the inlet port of the oil pump, an oil supply passage defined in the housing and interconnecting the low-pressure oil passage and the oil tank, for supplying oil pumped from the oil tank by the oil pump for circulation between the oil pump and the oil motor through the high-pressure oil passage and the low-pressure oil passage, and a return oil passage defined in the housing and connecting at least a portion of the low-pressure oil passage to the oil tank, for removing air trapped in the oil circulating between the oil pump and the oil motor.