The heat and mass transfer process between falling liquid desiccant film and air in parallel flow heat exchanger is investigated numerically. The governing equations with appropriate boundary and interfacial conditions describing the physical problem are derived. The control volume approach is used to predict the outlet conditions for both the air and the desiccant solution. The effect of inlet conditions, mass flow rates and channel geometry on the air cooling and dehumidification processes is also predicted. The average Nusselt and Sherwood numbers for air flow are correlated in terms of Prandtl number, Schmidt number and channel geometry. Typical numerical experiments showed good agreement of the present results with the available data in literature. Moreover, a parametric study is conducted to illustrate the general effects of various variables on heat and mass transfer processes in cooling and dehumidification of air.