For irrigation scheduling and better management of the water, estimation of irrigation water requirement of the crop is essentially required. The study was conducted for estimation of daily reference evapotranspiration (ETo) by FAO Penman-Monteith method using 30 years (1985-2015) mean meteorological data of two locations Sabour (zone III A) and Patna (zone III B) in Bihar. The crop evapotranspiration was estimated using crop coefficients of different crops like rice,kharif maize, wheat, rabi maize, green gram and summer maize at Sabour and Patna locations. Irrigation water requirement for different crops was estimated based on crop evapotranspiration and effective rainfall during the growing period of the crop. The mean annual reference evapotranspiration (ETo) was found 3.6 mmday1 at Sabour and 4.1 mmday-1 at Patna. The total crop evapotranspiration was maximum for rice crop in kharif season 546.0 and 607.3 mm at Sabour and Patna respectively and lowest for wheat crop in rabi season 212.9 mm and 243.3 mm at Sabour and Patna respectively. Net irrigation water requirement was maximum for summer maize366.3at Sabour and 508.2 mm at Patna for rice crop whereas lowest in kharif maize 67.5 and 161.4 mm at Sabour and Patna respectively