Abstract: A numerical simulation was performed to investigate two bubbles rising side by side in vertical channel using VOSET (Coupled Volume-of-Fluid and Level Set) method and N-S equations coupled with continuous surface force (CSF) model. Behaviors of coalescence between two identical bubbles predicted were in good agreement with experimental results reported in literature. Effects of surface tension on trajectories and velocities during rising process are investigated. Three types of motion were observed depending on surface tension:Coalescence, two bubbles repeatedly attracted and bounced against each other, bounced and separated. Without coalescence, trajectory shows symmetry about channel center line. Vertical velocities of both bubbles are almost the same while magnitude of horizontal velocity with opposite direction equals.