The segregation process of binary mixture with different density ratios in a granular bed is investigated in this paper. Particles that were 14% (by volume) heavier and 86% lighter put in a container subjected to an external vertical vibration. The velocity fields of both species were measured by an image-processing system. The segregation intensity was determined from the recorded images. It was found that the heavier particles tended to move towards the centers of the two convection cells of the lighter particles. The mixture with the greater density ratio could be segregated more completely. The increase in the vibration acceleration helps the segregation process.