A visual experimental investigation on Taylor bubble behavior of two-phase flow was carried out in vertical circular channels, with the length of 1000 mm and inner diameter of 1 mm, 2 mm, 3 mm and 6 mm. Deionized water was used as the liquid-phase working fluid, and air, nitrogen, argon and carbon dioxide were adopted as the gas-phase working fluids. The interface morphology of Taylor bubble was recorded by a high speed camera, and the variations of pressure drop and liquid-phase mass flux were measured simultaneously. The effects of different working conditions on the interface morphology of Taylor bubble were discussed in detail, and the effect of surface tension on the Taylor bubble behavior was investigated based on the Eo (Eotvos) number and the Mo (Morton) number. Also, the fluctuations of pressure drop and liquid-phase mass flux were analyzed in detail based on the Taylor bubble behavior. Finally, a model for prediction of the two-phase flow pressure drop in mini-channel was proposed, and this model was validated based on the experimental result with good agreement. (C) 2015 Elsevier Ltd. All rights reserved.