Void fraction measurement is very important in the thermal-hydraulic experiments. In this work, a new measuring method was proposed to detect the bubbles’ size and sketch by using the high-speed camera. To check the accuracy of this method, flow boiling condition in the vertical rectangular narrow channel was considered. Firstly, boiling phenomenon was observed in the test pipe. Then recognition and statistic of bubbles were performed to obtain the bubbles’ quantity, size and speed. The conclusion of this work is that the bubble images can be divided into nearly round bubbles and irregular bubbles and this method have a good performance in the bubble s’ recognition and statistic.Copyright © 2014 by ASME