Abstract: Response surface methodology was used to investigate the effects of temperature, thickness and time on the drying of water yam slices and to determine the optimised condition for hot air drying. The predominant falling rate drying regime was observed. Experiments were performed at air temperature of 60oC, 70oC and 80oC, slice thickness of 4, 6 and 8mm and drying times of 60, 165 and 270minutes. Based on response surface and desirability functions, the optimum conditions for water yam drying were: air temperature70OC, 74.9OC, slice thickness 6mm, 6.6mm and drying time 165minutes, 116.1minutes for untreated and treated water yam respectively. At this point, the predicted responses for drying rate were 0.000345kg/m2s, 0.000358kg/m2s respectively.