To understand the spatial variations of the desalination behavior, a mathematical model is systematically formulated to simulate a direct contact membrane distillation (DCMD) with the hollow fiber module for desalination. The model equation for the entire module is derived by integrating the mass, momentum and energy balances on both feed and permeate sides with the permeate flux across the membrane. The property variations of feed and permeate sides along the length of the membrane module are thoroughly simulated. Sensitivity of the permeate flux and the thermal efficiency along the fiber length to operating conditions is further investigated over a range of temperature and flow rate. It is shown that both permeate flux and thermal efficiency decrease along the fiber length as the feed gets cooled down. Although the velocity variations of both streams are not big, the flow rate ratio of the hot feed to the cold solution has a dramatic impact oil the distribution of the permeate flux and the thermal efficiency along the fiber length. The effect analysis Could potentially be applied to optimal design and scale-Lip of hollow fiber DCMD modules.