We recently defined a mixing index based on the concept of coordination number and used it to investigate mixing states of particulate mixtures at a particle scale. In this work, using Discrete Element Method (DEM) we simulate the mixing of mono-sized fine particles in a vertical cylindrical bladed mixer, and apply the proposed mixing index to asses the mixing process. To investigate the effect of cohesiveness between particles, the van der Waals forces are implemented in the DEM used in our previous work. Two dosages of particles equal in numbers are laid in the top-bottom arrangement as the initial state. The results show that there is a sudden deterioration in the degree of mixing when the cohesive force or the Hamaker constant Ha is larger than a certain value. However, the degree of mixing does not always decrease with the increase of Ha; in fact it improves at some large values of Ha.