Abstract: Accurate determination of reference evapotranspiration（ETo）is very essential to precise computation of crop water use.The FAO-56 Penman-Monteith method（FAO-56 PM）is the standard method recognized by the Food and Agriculture Organization of the United Nations for estimating reference evapotranspiration.Unfortunately，some of the weather variables，especially relative humidity，solar radiation，and wind speed are often missing，which could impede the estimation of ETo with the FAO-56 PM method.To overcome the availability of climatic variables，assessing the accuracy and usefulness of Hargreaves method for Pusa Observatory located in the campus of Rajendra Agricultural University，Pusa is important.The Hargreaves equation showed a regular overestimation of the region，hence the method was discarded for Pusa location.Modifications were made in Hargreaves coefficient using statistical regression analysis and were calibrated from 0.002 3 and 17.8（Hargreaves equation）to 0.001 62 and -3.039 39 respectively.Both the Root Mean Square Error（RMSE）and the Mean Bias Error（MBE）of the comparison between ETo computed by adjusted Hargreaves equation and that computed from FAO-56 PM method were less than 1.00 and 0.21 with a minimum of 0.77 and -0.11 mm/d，respectively leading to small errors in the ETo estimates.R2，X2 -test，and index of agreement values confirmed strong relationships among data for the year from 1998 to 2006.It indicated that ETo computed by adjusted Hargreaves equation（an average value of ETAHG 3.71 mm/d）and that computed from FAO-56 PM method were in close agreement at Pusa region.