A method for calibrating a superheat sensor (5) for a refrigeration system is provided. The method comprises the following steps. Increasing an amount of liquid refrigerant in the evaporator (1), e.g. by increasing an opening degree of the expansion valve (3). Monitoring one or more parameters, e.g. the temperature of refrigerant leaving the evaporator (1), said parameters reflecting a superheat value of the refrigerant. Allowing the value of each of the parameter(s) to decrease. When the value(s) of the monitored parameter(s) reaches a substantially constant level, defining the superheat value corresponding to the constant level to be SH=0. The superheat sensor (5) is then calibrated in accordance with the defined SH=0 level. When the parameter(s) reaches the substantially constant level it is an indication that liquid refrigerant is allowed to pass through the evaporator (1), and thereby that the superheat of the refrigerant leaving the evaporator (1) is zero. Calibration can be performed on site, and it is therefore not necessary to calibrate the sensor (5) at the manufacturing facility. Thereby it is no longer required to match calibration information with a specific sensor.