In this work, the characterization of the two-phase flow of liquid−liquid in millireactors made by additive manufacturing was carried out. Redistilled water and dodecane, which represent immiscible fluids in the petrochemical, pharmaceutical and other industries, were chosen as model liquids. The influence of reactor channel diameter, type of millimixer, liquid flow rate and phase flow ratio on the flow type was determined. A total of 84 experiments were conducted in three types of millireactors with different millimixers (T and Y) and channel diameters (2,5 mm and 1,5 mm). Flow types and dimensionless features were examined. Based on the results, it is possible to conclude the following: the shape of the millimixers does not have a significant influence on the establishment of the flow regime as it is in the case of microreactors. Flow types and dimensionless features (Reynolds, capillary, Webber and Bond numbers) were examined. By calculating the Bond number feature for millireactors with a tube diameter of 2.5 mm, the influence of the gravitational force was observed; while with a pipe diameter of 1.5 mm the forces of interfacial tension and viscosity dominate. Geometry and the presence of bends influence the stability and flow regime in millireactors. Fused filament fabrication, FFF, has proven to be a fast, simple and inexpensive method of manufacturing millireactors. The obtained results represent a contribution to the use of millireactors for systems of immiscible liquids , on the basis of which it is possible to predict the type of flow and optimize chemical processes (synthesis, extraction, separation).