When both a gas and liquid are simultaneously introduced from an injector into a vertical mini-channel, the hydrodynamic pattern in the channel depends on the construction of the injectors. To evaluate the effect of the injector on the two-phase flow pattern, direct injection has been compared to a co-flow pre-injection and a cross-flow pre-injection in recent study. For the three different injections, the flow pattern regime maps, which are described as the relation between the superficial liquid velocity and superficial gas velocity, were developed. The direct injector realized better dispersion of uniform bubble flow, various flow patterns and stable slug flow than the other injections. To clarify the two-phase flow dynamics in a vertical mini-channel attached to a direct injector, the frictional pressure drop has been measured and illustrated by a contour. The frictional pressure drop through a mini-channel was correlated using the equation of Chisholm. The two-phase friction multiplier was plotted versus the Lockhart-Martinelli parameter. The constant of the Chisholm equation has been evaluated for the two-phase flow such as bubbly flow and slug flow. By using the equation, the frictional pressure drop through a mini-channel can be well predicted.