Wake characteristics of a rising air and vapor bubble through stagnant and flowing deionized water in a vertical narrow rectangular channel were investigated using particle image velocimetry (PIV). Influence of inlet average fluid velocity on bubble behavior was discussed. The results showed that relative terminal velocities of air bubble under different inlet average fluid velocities were almost equal to the terminal velocity in stagnant fluid, and terminal rising velocities of single air bubble and vapor bubble are very close under adiabatic boundary condition. As flow velocity increased, wake structure simplified. Disturbances in the wake of the vapor bubble were more noticeable.