In the present study, the local superficial gas velocity was calculated as a sum of the contributions of the size and frequency, or the rise velocity and gas holdup, of the individual bubbles rising vertically through the probe. A correction factor alpha(i) for any detected bubble i was needed to take into account the probability that some of the bubbles vertically approaching the probe might not be detected due to the chaotic behaviors of bubbles and liquid. The reciprocal of alpha(i) was regarded as the bubble detection efficiency of the probe. The value of alpha(i) was assumed to be specific to the gas-liquid flow pattern, i.e., the bubble, churn-turbulent or slug flow. As a result, the effects of the various operating conditions on the radial distribution of the gas velocity were determined. The distribution was found to be more remarkable for the higher superficial gas velocity and for the smaller column diameter.