Provided are a washing machine and a method of controlling the washing machine. The method includes measuring a time required for an amount of washing water supplied to a washing tub to reach a predetermined reference amount, determining a water pressure of the washing water in accordance with the measured time, and varying a water supply time in accordance with the determined water pressure. The water supply time can be adjusted according to the water pressure, it can be prevented that the washing performance is varied whenever the wash water is varied. Thus, the washing performance can be improved regardless of the water pressure.