Absorber is the most important component in LiBr-H2O absorption systems as its operating performance directly influences the performance of the whole system. In simulating absorption heat and mass transfer in horizontal tube absorber, many authors, for simplicity, make some assumptions. One of these assumptions is that the thermo-physical properties of the solution are constant across the absorber. In this paper, LiBr-H2O horizontal tube absorber was simulated with constant and variable thermo-physical properties and a comparison between the performance parameters (solution concentration, solution temperature, absorber heat duty and rate of vapor absorption) was made. Two solution flow rates are used in this simulation (Γ=0.0375kgs −1 m −1 and Γ=0.025kgs −1 m −1) and a computer program was developed to simulate the two cases (constant and variable thermo-physical properties). The comparison results show that, in case of variable thermo-physical properties, the heat duty and vapor absorption rate are less than those in case of constant thermo-physical properties whereas solution concentration and temperature are slightly greater. Also, it is found that increasing solution volume flow rate (Γ)increases the difference in heat duty and absorption rate and slightly decreases the difference in solution concentration and temperature.