Liquid-liquid two-phase flows are encountered in a substantial number of industrial processes. Shape and distribution of the mobile liquid-liquid interface, the base on which flow patterns are usually categorized, significantly affect pressure drop and heat and mass transfer rates. Therefore, accurate prediction of flow patterns is of great importance in many industrial applications. Despite their importance, however, liquid-liquid two-phase flow phenomena are not as well understood as gas-liquid systems. To gain a better understanding of liquid-liquid two-phase flow behavior, particularly under high pressure, flow patterns of n-hexadecane-CO2 liquid-liquid two-phase flow are experimentally investigated in vertical stainless steel pipes of inner diameter 1.5mm and lengths of 68 and 500mm under pressures from 1500psig to 4300psig using a high pressure flow visualization system. This work is the first attempt to observe and discuss different flow patterns of liquid-liquid two-phase flow under high pressures. Bubbly flow, slug flow, plug flow, and annular flow are observed in both pipes. Flow pattern maps are constructed for both cases by plotting data in flow rate ratio and pressure coordinates. The results demonstrate significant effects of flow rate ratio, pressure, and pipe length on flow patterns. Movies of different flow patterns will be shown in the presentation.