A laundry treating apparatus may include a washing agent supply unit supplying a liquid washing agent into a tub through a washing agent supply passage between a storage container and the tub, a residual quantity detecting unit to detect a residual quantity of the liquid washing agent, a laundry quantity detecting unit to detect a laundry quantity, and a controller controlling the washing agent supply unit to supply the liquid washing agent by adding a quantity of the liquid washing agent corresponding to the laundry quantity with a quantity of the liquid washing agent corresponding to the washing agent supply passage, when the liquid washing agent is initially supplied after detecting a residual quantity shortage of the washing agent, whereby an accurate quantity of the liquid washing agent can be supplied upon an initial supply after the detection of the residual quantity shortage.