A solar tunnel drier is optimised for drying of chilli in Bangladesh. The simulation model was combined with the economic model of the solar tunnel drier and adaptive pattern search was used to find the optimum dimensions of the collector and the drying unit. Two optimum designs are obtained. For design-1, both collector and drying unit are 14.0 m long and 1.9 m wide and for design-2, both collector and drying unit are 13.0 m long and 2.0 m wide. Both the collector and drying unit of basic mode drier are 10.0 m long and 1.8 m wide. The capacity of optimum mode driers is higher than the basic mode drier and achieves a cost saving of 15.9%. The pay back period of the basic mode drier is 4 years and optimum mode drier is about 3 years. Sensitivity analysis showed that the design geometry is sensitive to costs of major construction materials of the collector and air temperature in the drier. (C) 2004 Published by Elsevier Ltd.