The present invention relates to system for calibration of gas sensors and more particularly, the present invention relates to a system for characterization and calibration of bosch zirconium oxide based modified lambda sensors used in oil fired boiler stack gas oxygen measurement.The system comprises a muffle furnace attached to a gas selection mechanism, wherein said muffle furnance is being provided with the gas sensor wich is to be characterized and a temperature sensor; The gas selection mechanism supplies a predetermined amount of an air containing a predetermined quantity of the gas to be detected to the muffle furnace; the gas sensor and the temperature sensors being coupled to a micro converter through a buffer and offset amplifier mechanism and a signal conditioning mechanism respectively; an output from the micro converter is coupled to a display unit for displaying the values measured by the gas and the temperature sensors.