Three‐dimensional numerical simulations were performed to study the liquid flow inside split T‐type micromixers, investigating the effects of various operating and design parameters on the mixing process. While for small Reynolds numbers, the mixing efficiency of a split T‐micromixer is larger than that of a simple T‐micromixer, it was found that, when Re = 100, mixing is further enhanced by the formation of vortex pairs. This is not only a function of the flow rate, but it depends also on the geometry of the device and on the ratio between the flow rates in the inlet sub‐channels. In general, at a given flow rate, vertical splits induce a much better mixing than horizontal splits, while the reverse may occur for strongly unequal inlet flow rates.