A motor/generator cooling system of a hybrid vehicle that has a first and second motor/generator disposed in an axial direction in a transmission, may include a cooling pipe that may be disposed in an axial direction along an upper and outside of the first and second motor/generator in the transmission, a rear cover that may be connected to a downstream side of the cooling pipe such that the cooling pipe may be fixed thereto, an input supporter that may be connected to an upstream side of the cooling pipe, forms a cooling oil passage, and rotatably supports a rotor of the first motor/generator, and a case that the rear cover and the input supporter may be mounted thereon, an oil filter connected to a hydraulic pump may be mounted therein, a penetration hole may be formed therein, and the cooling pipe penetrates the penetration hole to be fixed thereto.