High-speed planetary mixers with no impellers have been used to blend various types of industrial powders and viscous fluids rapidly and uniformly by means of centrifugal agitation resulting from the combined rotating and revolving motion of a mixing vessel. However, there is still a lack of in-depth understanding of their mixing dynamics at both meso and macro scales. This paper presents a discrete element simulation of the effect of cohesive powder rheology on mixing performance in a blade-free planetary mixer at particle scale. The interparticle cohesive forces were computed using the Johnson-Kendall-Roberts (JKR) model where the surface energy of each particle is a control parameter for the rheological behavior of the bulk powder. A series of mixing simulations were conducted for cohesive and non-cohesive particles in a planetary mixer. The degree of mixing in each simulation was evaluated with the performance parameters such as the mixing index, mixing time, and relative particle fraction. The simulation results showed that cohesion retards the particle mixing rate, but an increasing cohesion does not deteriorate mixing performance in a high-speed planetary mixer. The simulation method adopted in this study can be used in optimizing the design and operation conditions of a planetary mixer.