This paper investigates the air bubble size and its transitions in a horizontal tube of 700 mm. The tube is assembled with a bubble generator. The present study varies the flow-rates of water and air. The data collection mainly uses a high speed camera to capture the bubbles at different distances along the horizontal tube. Image processing uses MATLAB for evaluating the bubble size. The data interpretation uses Yw dimensionless parameter in representing the level of the bubbles vertical rise in the horizontal tube. The bubble generator produces bubbles in the range of 0.8-3.1 mm at the inlet of horizontal tube using water flow-rates (Qw) of 120-170 litre per min (LPM) and air flow-rates (Qa) of 2-10 LPM. The diameter of the bubble increases as it moves horizontally from the inlet to the outlet of the horizontal tube and this finding is statistically significant. The bubble vertical rise is visually obvious and has been quantified along the horizontal tube. The YW increases axially from 41 % to 89 % from the inlet to outlet of the horizontal tube. The YW increases when either the Qa or Qw is reduced. The findings from this study give a fundamental insight into bubbles dynamics behaviour in its horizontal transition.