The design and operation of Electrostatic Precipitator depends on the properties of the coal burned and fly ash generated in the boilers of the power plant. The properties of the coal used in different power plants cross the India vary widely, but most of the times the sulfur contents are low in the coals. As a result the resistivity of the fly ash is in general, very high and it leads to develop back corona at the collecting electrodes even at comparatively low current densities flowing through the different fields of ESP. All this results in detorieting the Collection efficiency of ESP and large size of ESP is required to meet the emission standard for similar level of power generation elsewhere. Knowledge of electrical resistivity is essential for sizing of ESP and to determine a strategy to improve its performance. A special facility has been developed at IIT Delhi to investigate the variation of resistivity at different temperature and moisture levels under simulated conditions those exiting in side an ESP. A series of experiments have been conducted on the fly ash samples obtained from different power plants in India. Empirical relations developed by Bickelhaupt, which are based on chemical composition of fly ash for calculating the electrical resistivity, are used to calculate as theoretical value for given experimental conditions. New empirical relations based on experimental results and operating conditions have been developed to provide better agreements.