RTM (Resin Transfer Moulding) is a closed mould process in which matrix resin is injected into a mould filled with a fibre preform. There are many RTM process parameters influencing the quality of the final product. The objective of this work was to study resin injection pressure. For this purpose, an amine curing epoxy resin was injected, using a small laboratory size RTM machine, into a flat mould filled with woven carbon fibre layers. Five different resin injection pressure levels; 1, 2, 3, 4, and 5 atm (101, 203, 304, 405, and 506 kPa) were used to produce composite sample plates. Specimens for mechanical testing and microscopic examination were extracted from various parts of the plates. Generally, it was observed that an injection pressure level of 2 atm (203 kPa) resulted in the best mechanical performance in the specimens, mainly due to better fibre impregnation by the uniform resin flow, minimising void formation. Increasing the resin injection pressure level above 2 atm (203 kPa) resulted in differences between "macroflow" (around fibre bundles) and "microflow" (through the fibres) leading to non-uniform resin flow and void formation, hence a reduction in the mechanical properties.