In the present work, unidirectional flax fibre reinforced epoxy green composites were prepared with hand lay-up process. Efforts were made to improve the mechanical properties of the composites to meet the requirements for civil structural application. Mercerisation under stretch was found to be an effective way to enhance the interfacial bonding performance between flax and epoxy matrix, leading to the increased flexural strength and modulus. The incorporation of organoclay to epoxy matrix can further increase the interfacial strength and modulus remarkably. With optimised mercerisation time, the flexural strength and modulus of the unidirectional flax/epoxy composite of fibre content about 25 vol. % reach 183 MPa and 5.2 GPa respectively, while the incorporation of 2% of organoclay can further increase the flexural modulus and the short beam shear strength in transverse direction by about 30%. The preliminary study indicates the promising mechanical properties of the flax fibre-based green composites for civil structural application.