Isothermal degorption kinetics of arachidic acid molecules in Langmuir-Blodgett multilayers is studied with attenuated total reflection method. A simple model has been proposed to analyse the thermal desorption process of the multilayer system. The calculated desorption energy between two neighboring molecular layers with hydrophobic ends against hydrophobic ends was found to be 19 kcal/mol; while with hydrophilic ends against hydrophilic ends, the energy was found to be 27 kcal/mol.