Disclosed are an apparatus and a method for liquid-treating a substrate. The substrate treating apparatus includes a liquid supply unit configured to supply a treatment liquid in which a first liquid and a second liquid are mixed, onto the substrate unit, wherein the liquid supply unit includes a nozzle configured to discharge the treatment liquid, a first liquid supply line supplying the first liquid to the nozzle, and a second liquid supply supplying the second liquid to the nozzle, and the nozzle includes a body having a mixing space in which the first liquid and the second liquid are mixed and a buffer space extending from the mixing space, in the interior thereof, and a collision member located in the buffer space and configured to decrease a flow velocity of the treatment liquid supplied to the buffer space.