Bubbly flow in water involved thousands of bubbles in a vertical pipe is numerically simulated in this paper. The motions of the bubbles are tracked using a Discrete Phase Model (DPM). The interactions among bubbles are simulated through the model of discrete element method (DEM). The effect of bubble diameter on the bubble flow trajectories is studied. Comparisons are made on the flow field with and without considering bubble-bubble collision. It is found that the flow fields become fluctuated when the bubble diameter is above a critical value. The larger the bubble diameter, the higher the fluctuation is. However, as the bubble diameter is smaller than 3 μm, the flow field acts as no bubbles inside. The inclusion of the bubble interactions makes the flow field more fluctuated. Drag force coefficients with and without considering bubble-bubble collisions are compared.