A method of determining a state of cooling water is provided. The method includes operating, by a controller, a driving motor of a cooling water-circulating pump that is configured to circulate cooling water at a fixed current, a fixed torque, or a fixed power. In addition, the controller is configured to calculate an average rotation speed of the driving motor for a preset first period of time during the operation of the driving motor. Whether the circulation state of the cooling water is normal is determined based on an error between the calculated average rotation speed and a preset reference rotation speed.