To maximize the desalination performance of both productivity and thermal efficiency, model-based optimal design of a hollow fiber direct contact membrane distillation (DCMD) module is proposed. The mathematical model equations for the entire module are derived by integrating the permeate flux across the membrane with the mass, momentum and energy balances on both feed and permeate sides. The property variations of feed and permeate sides along the length of the membrane module are simulated. It is found that there is a trade-off between the permeate flux (productivity) and thermal efficiency. The trade-off depends on not only the operating variables, such as the temperature and the flow rate of the cold solution, but also the fiber dimensions, such as the fiber length and the packing density. Based on the above decision variables, optimization of the hollow fiber DCMD module is carried out using the Pareto genetic algorithm. It is demonstrated that the optimal values of the inlet cold flow rate and the fiber length are more sensitive than the module packing density. With the synchronous increase in the inlet cold flow rate and the fiber length, the productivity increases at the cost of higher thermal efficiency. (C) 2008 Elsevier B.V. All rights reserved.