Application of discrete vortex elements for numerical calculations on propeller/duct system is presented. The flexibility and reliability of the computed results by the method is discussed in comparison with the other methods and experiments. The following items are investigated in this paper : i) The possibilities of correct modelling on propeller/duct geometry and real physical phenomena ii) The two kinds of interaction model between propeller and duct ; a complete lifting surface (direct interaction model) and an infinite number of blades (indirect interaction model) in place of the propeller toward the duct lifting surface iii) The correlations between measured and calculated circulation, pressure and open water characteristics on propeller/duct systems iv) The influence of relative positions between propeller and duct on duct thrust components and circulation distribution The advantages of the direct interaction model have been obtained on saving computer time with reasonable accuracy for propeller/duct performance and circulation distribution on a blade. On the other hand, the indirect interaction method has become known to be useful to obtain the good agreement with experiments on pressure distribution on duct surface.