Abstract: Long term changes of crop water requirement (CWR) and reference evapotranspiration have a significant effect on agricultural production and water resources management. Based on historical meteorological data, reference evapotranspiration (ETo), crop water requirement (derived by multiplying ETo by crop coefficient) and crop water deficit (the difference between crop water requirement and rainfall) were calculated using modified FAO Penman-Monteith equation. The change trend was then analyzed for winter wheat and maize in Hebei Province for the period 1965 ~ 1999. Results show a significant decrease in reference evapotranspiration and crop water requirement. Decrease in CWR is 26 mm per decade for winter wheat and 9.7 mm and 9.9 mm for summer and spring corn respectively. There are differences in water deficit among different crops but the average increase in water deficit is insignificant, which is respectively 345 mm, 20 mm and 29 mm per year for winter wheat, summer corn and spring corn.