Disclosed is an apparatus for treating a substrate. The apparatus includes a substrate support unit that supports the substrate, a nozzle unit that dispenses a treatment liquid onto the substrate supported on the substrate support unit, and a liquid supply unit that supplies the treatment liquid to the nozzle unit. The liquid supply unit includes a main supply line that is connected to the nozzle unit and that supplies the treatment liquid to the nozzle unit, wherein the treatment liquid is prepared by mixing a chemical with DIW at a first temperature and DIW at a second temperature higher than the first temperature, and temperature of the treatment liquid is adjusted by regulating a flow rate of the DIW at the first temperature and a flow rate of the DIW at the second temperature without a separate heater.