Thermodynamic characteristics of the condensation of nonazeotropic refrigerant mixtures (NARMs) inside tubes are discussed. A one-dimensional model of the condensation of NARMs in a horizontal tube is then proposed. An annular flow pattern is assumed in this model. Energy and mass balances, as well as heat and mass transfer equations are presented for the axial vapor flow and the condensate liquid flow respectively. Simulation has been conducted by using a double pipe condenser with R12/R114 mixture and cooling water in counter-current flow. The composition of the axial vapor flow is found to approach that of the more volatile component in the down stream of the condenser. The diffusion flux in the liquid condensate is found to impose very little effect on the local condensed mass flux. Contrarily, the counter diffusion of the more volatile component in the vapor side of the vapor-liquid interface is found to play an important role. Simulation results are compared with experimental data, and good agreements are obtained.