A direct numerical simulation(DNS)was conducted for a particle-laden turbulent flow in a vertical channel. Particular attension was focused on the turbulence modulation through the Reynolds stress, rather than the wake-induced perturbation. We simulated downward flow as well as upward flow to observe the influence of the direction of particle forces to the mainstream. Comparison between our calculation with a corresponding experiment showed qualitatively reasonable agreement. Reynolds shear stress was increased in a downward flow while reduced in an upward flow according to the momentum balance change due to the particle-induced force. As a result, the production of turbulence energy was increased/decreased in a downward/upward flow. It was also affected by particle distribution. We therefore suppose the turbulence modulation without vortex shedding from particles could be predicted if the momentum exchange between two phases and the distribution of particle concentration were given accurately.