The normal restitution coefficient is one of the important parameters in the process of particle collision, which is widely used in discrete element modelling (DEM) of gas-solid multiphase systems. In order to get more accurate results of the simulation, more accurate parameters for DEM modelling are needed. The determination of the normal restitution coefficient for the dry condition is relatively straightforward; however, the unpredictable behaviour of this coefficient under humid conditions makes its determination more complex. This paper presents an experimental system to determine the normal restitution coefficient using a high-speed camera. Experiments were performed to measure the incident and rebound velocities of three kinds of fly ash particles with a diameter of approximately 7 mu m. Furthermore, the Stokes number is the key parameter to calculate the relationship between dry and humid normal restitution coefficients. The existing theory is suitable for analysing the collision with a thin liquid film; however, the method is not applicable to the humid conditions. Therefore, a modified Stokes number is proposed to calculate the normal restitution coefficient under these conditions. The normal restitution coefficient is calculated by the modified formula, which is in a good agreement with the experimental results.