An attempt was made to predict numerically the erosion rate of wall surfaces of a gas-solid suspension flow duct in order to establish a prediction method with wide applicability. First, the erosion rate due to a single particle collision was quantified using equations by Finnie and Bitter. The empirical constants for these equations were determined experimentally. Then, the expressions obtained were incorporated into a mathematical model based on the Eulerian description for the continuous phase as well as on the Lagrangian description for the particulate phase. In order to demonstrate the prediction accuracy, an attempt was made to reproduce the actual erosion rate observed on the surfaces of a square cross-sectioned L-type suspension flow duct. Fairy good agreement was obtained between the prediction and the measurement.