This work presents a comprehensive mathematical model of the performance of a cereal thresher. The essence of this work is to improve on the performance of grain thresher through a model that will optimize the performance characteristics (velocity, crop feed rate, concave clearance, threshing drum impact force, crop flow rate) of a thresher and minimize its grain damage or losses. To achieve this, equations were derived which describe the threshing rate, threshing loss, grain damage, efficiency and threshing capacity. The equations were used to simulate and optimize the threshing performance of the model. The model was validated with published threshing performance data and it was found generally to fit well, taking R2 values equal to or greater than 0.9 which is highly significant (α= 0.000). The developed model enables optimal parameters to be selected for a design in which the objective function (grain damage or losses) is minimized. At 16% moisture level, minimal loss is 3.6792% and efficiency of 92.32% while the performance characteristics are v= 9m/s, Q= 0.12 kg/s, c= 0.015 m,