A control system of a power transmission system of a vehicle is provided. The power transmission system includes an electric motor for running the vehicle, and a mechanical speed change mechanism. The control system includes an electronic control unit. The electronic control unit is configured to: perform regeneration control of the electric motor for running the vehicle during coasting of the vehicle, such that regenerative torque of the electric motor for running the vehicle provides regenerative torque produced according to braking operation; perform shift control of the mechanical speed change mechanism according to a predetermined relationship; determine whether a rate of change of the regenerative torque is within a predetermined range; and when a downshift of the mechanical speed change mechanism is determined during coasting, execute the downshift under a condition that the rate of change of the regenerative torque is within the predetermined range.