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The Penman-Monteith (PM) equation was introduced as one of the most reliable equations to determine crop ETc, without using crop coefficient or ETo values. In this study, the PM equation was evaluated using lysimeters in a semi-arid region for wheat and maize. Different equations for aerodynamic resistance (r(a)) and canopy resistance (r(c)) were tested in the PM equation and they were ranked using statistical analysis. It was shown that the combined method of r(a) and r(c) in FAO-56 does not lead to a good prediction of ETc for wheat and maize in comparison with the lysimeter-measured data. The results indicated that a modified equation for r(c) was the most accurate method for both wheat and maize. Using this equation, the suggested model of FAO-56 and another investigation for r(a) led to the best results for wheat and maize, respectively. Furthermore, it was shown that the previously modified equation for r(c) was newly modified as a function of vapor pressure deficit (VPD) and the results were as accurate as before. Therefore, an equation as a function of VPD can be used when solar radiation (R-n) is not available easily.