The transient split behavior of slug flow units at an impacting micro-T-junction is studied by the VOF method. The micro-mechanism for the phase split of slug flow at the impacting micro-T-junction is disclosed. The effects of the characteristics of slug flow units on the split of gas bubbles and liquid slugs are studied. These characteristics include the Ca number, the length/length ratio of the gas bubble to the liquid slug, as well as the surface tension. The split of a gas bubble at the T-junction shows obstructing, non obstructing modes, or a combination of them. It is found that the length of the gas bubble and the Ca number of the inlet flow play key roles in determining the split modes. The split ratio of the continuous flow shows a notable impact on the split of a gas bubble as its length is 2 diameters less than the tube. At a critical liquid split ratio, the bubble shifts from a fissionable one to a non-fissionable one. A longer gas bubble requires a higher liquid split ratio to be shifted from a fissionable state to a non-fissionable state, which then would lead to a wider linear region on the phase split curve. The length ratio of the gas bubble to the liquid slug slightly affects the split behavior of the unit, by causing a merge of the consecutive gas bubbles at the junction and thus the change of split mode. A lower surface tension facilitates the split of the gas bubbles and thus reduces the maldistribution of the slug unit. Some of these transient features have been directly reflected on the averaged phase split data obtained from our previous experiments. (c) 2017 Elsevier Ltd. All rights reserved.