Granular materials exhibit convective motions in a confined box under external vibration. Particles move upward in the center and flow downward along the side walls. This paper experimentally studies the phenomena of convection cells in a two-dimensional box with 3-mm glass beads under different vibration intensities. Using image processing technology and particle tracking method, the velocity fields of the convective granular beds were measured and analyzed at different vibration conditions. The vertical and horizontal velocity profiles at different locations were investigated. The convection cell centers could be determined from the velocity profiles. The convection flow rate was calculated and had a power law dependence on the amplitude of the vibration velocity.