The rapid growth in microelectronic devices and mini refrigeration systems calls for the utilisation of multiphase flow boiling that provides heat transfer enhancement compared to single-phase cooling. Due to its complex behaviour, the applications of flow boiling in the mini/microsystem depends on various factors such as the operating condition, channel geometry, and liquid properties. Although there has been a great effort to cover various parameters influencing flow boiling, only a few studies have investigated high aspect ratio microchannels while at the same time considering the effect of channel orientation. In the present study, flow boiling characteristics in a high aspect ratio microchannel at different channel orientations are investigated. Here, a rectangular borosilicate channel, with an aspect ratio (width to height) of 10 and a hydraulic diameter of 909 µm, was used, while hydrofluoroether (HFE−7000) was chosen as the working fluid. The mass flux was set at 42 kg m⁻² s⁻¹, and the heat flux was varied between 5 kW m⁻² and 24.3 kW m⁻². Two different flow orientations were considered, horizontal flow and vertical upward flow. The result shows that the wall temperature distribution is affected by both heat flux and flow orientation. In terms of thermal performance, the heat transfer coefficient increases with the overall vapour quality defined as x and reaches a peak at around x = 0.1. After that, it experiences a gradual decrease. The influence of heat flux in heat transfer can be strongly observed in the case of low heat flux and becomes less important at high heat flux. It is also found that the vertical upward flow, due to the bubble movement ease by gravity and the liquid film behaviour, performs better compared to the horizontal flow.