This paper describes three-dimensional numerical simulation of double helical ribbon agitators using discrete element method (DEM). Double helical ribbon agitators have been used to blend powders, it is very important to know the mixing characteristics for scale-up of agitation system. To validate the computed results experiments were carried out with a 30 L cold model. Circulation time and mixing time of particles in the agitator and the shape of vortex predicted by this simulation agreed well with those obtained by experiments. Further, to evaluate the relation of scale up law based on mixing time, some cases of simulation, which volumes were both 90 L and 150 L, were performed. The results show the effect of scale-up factor on the mixing time strongly depends on Fr number.