Pneumatic conveying is an important technology in industries to transport bulk materials from one location to another. Different flow regimes have been observed in such a transportation process depending on operational conditions, but the underlying fundamentals are not clear. This paper presents a three-dimensional numerical study of vertical pneumatic conveying by a combined approach of discrete element model for particles and computational fluid dynamics for gas. The approach is verified by comparing the calculated and measured results in terms of particle flow pattern and gas pressure drop. It is shown that flow regimes usually encountered in vertical pneumatic conveying and their corresponding phase diagram can be reproduced. Then forces governing the behavior of particles, such as the particle-particle, particle-fluid, and particle-wall forces, are analyzed in detail. It is shown that the roles of these forces vary with flow regimes. A new phase diagram is proposed in terms of the key forces, which can successfully identify dilute-phase flow and dense-phase flow in vertical pneumatic conveying.