The objectives of the study are to predict the crop coefficient for Maize using watermarks gypsum blocks and atmometer apparatus during the growing stages, 2013, compare the estimating values with Food and Agriculture Organization in United Nations (FAO) and local study work. Thestudy work was conducted in the Al-Yusufiya field Township, 30km south of Baghdad under semi- arid weather conditions. The watermarks and atmometer were used to measure crop evapotranspiration and reference evapotranspiration, respectively. The predicted average value of crop coefficient for initial, develop, mid and late of seasons were: 0.14, 0.6, 0.91 and 0.57, respectively. The results from the comparison between the estimating crop coefficient and FAO values showed that the average values of relative error was36.29%and with mean absolute error was0.27. Moreover, root mean square difference was0.32.Additionallythe comparison between the estimating crop coefficientswith the local study values showedthat the average values of relative error was33.57% and with mean absolute error was 0.24. Moreover, the root mean square differencewas 0.29.