This research aims to study the internal flow visualization of a miniature loop thermosyphon for several ratios of the internal diameter of the condenser section to the internal diameter of the evaporator section and temperature. The miniature loop thermosyphon will separate gas and condensed liquid, which flow in opposite directions. This not only prevents flooding and drying out, it also increases heat transfer efficiency. The miniature loop thermosyphon consists of three sections, namely evaporator, adiabatic, and condenser sections. The return liquid channel is connected between the condenser and evaporator to convey the condensed liquid. The miniature loop thermosyphons are designed at three internal diameters of the condenser section to the internal diameters of the evaporator section ratios, which are 1.4, 1.6, and 1.8. The evaporator section consists of a copper tube with an internal diameter of 14 mm and a length of 135 mm. The adiabatic section consists of a glass tube with an internal diameter of 14 mm and a length of 135 mm. The condenser section consists of a copper tube with internal diameters of 19, 22, and 25 mm and a length of 135 mm. The return liquid channel consists of a glass tube with an internal diameter of 2.4 mm and a length of 135 mm. Ethanol was used as the working fluid, filled at 50% of internal volume of the evaporator section. It was found that the miniature loop thermosyphon can separate gas and condensed liquid that flows in the opposite direction at all internal diameter of the condenser section to the internal diameter of the evaporator section ratios. As the internal diameter of the condenser section to the internal diameter of the evaporator section ratio increases, the flow of gas and liquid is changed from a violent bubble flow to a churn flow, the velocity at liquid return channel decreases, and the heat transfer decreases.