⁸⁵Kr a fission product noble gas is released into environment through stack at reprocessing plants during dissolution of spent fuel. Dual GM tube based online monitoring system having efficiency 0.5% for ⁸⁵Kr is used for quantification of ⁸⁵Kr activity released through stack. Efficiency calibration of online monitoring system requires use of standard 85Kr source which is not readily available. As per safety regulations periodic efficiency calibration of Krypton monitoring system is required to be done at least once a year. This paper presents an indigenously developed methodology for efficiency validation of krypton monitoring system using portable HPGe gamma spectrometer.