In a substrate processing system according to an exemplary embodiment, gas supply units are configured to supply gases to chambers through first gas flow channels thereof, respectively. Chamber pressure sensors are configured to measure pressures in the chambers. A second gas flow channel is connected to the first gas flow channel of each of the gas supply units. A reference pressure sensor is configured to measure a pressure in the second gas flow channel. In a method according to an exemplary embodiment, each of the chamber pressure sensors is calibrated by using a measurement value thereof and a measurement value of the reference pressure sensor which are obtained in a state where pressures in a corresponding chamber, the first gas flow channel of a corresponding gas supply unit, and the second gas flow channel are maintained.