A smart glass transmittance control system includes: a smart glass that decreases transmittance of the smart glass in response to the quantity of light introduced and increases the transmittance of the smart glass when electric power is applied; a power supply unit to supply the electric power to the smart glass; a control unit to control supply of the electric power from the power supply unit to the smart glass in order to control the transmittance of the smart glass according to a user request. In particular, the control unit controls the supply of the electric power from the power supply unit to the smart glass based on the driving environment of a vehicle, the quantity of light from an external light source, or the driving environment condition of the smart glass.