All the works so far on simulation of grain drying by natural convection of air presents conflicting reslts on the drying time forecast. In this work the models of Hukill, Thompson and Michigan are tested in same type of dryer. Appropriate equations are developed to correct the drying time, starting form the models of Thompson and Hukill. The solution of the equations of michigan model is developed on fintie difference and similarly and some speical technuques are utilized on the Thompson model. 53 test results are utilzed here to validate the simulation model of drying.