The mechanical properties of fibre-reinforced composites depend on the properties of the fibre matrix interface, where stress concentrations prevail. In situ growth of carbon nanotubes (CNTs) on the surface of silica fibres produces “hairy” (or “fuzzy”) fibres, incorporating both micrometre and nanometre reinforcement length scales. Simulations of neighbouring CNT-grafted-fibres carrying CNTs with different lengths, perpendicular to the fibre surface, showed a reduction in radial and shear stresses at the fibre matrix interface. Stress concentrations are predicted to shift from the fibre matrix interface to the end of grafted CNT forest, effectively increasing the fibre diameter, resulting in higher stress concentrations in the resin rich region between the hierarchical fibres for longer CNTs [1]. While this mechanism has yet to be demonstrated experimentally, the introduction of grown CNTs has imbued thermal and electrical conductivity [2], and improved interfacial shear strength, albeit with reduced primary fibre tensile strength [3]. Here, the continuous production of CNT-grafted quartz fibres was performed in an open chemical vapour deposition (CVD) reactor with continuous in line catalyst deposition. Highly graphitic CNTs with controllable lengths ranging from 0.1 to 20 µm were grown on the quartz fibre surface by adjusting the reduction and growth times, with shorter fibres growing homogeneously and longer CNTs growing in a splayed “Mohawk” manner. The effect of CNTs length (and thus microstructure) upon the mechanical properties of CNT-grafted-quartz-fibre/epoxy composites was investigated through single fibre pull-out and tensile tests. This study not only allows direct comparison to the computational predictions on hairy fibres, but provides a route towards continuously grown CNT-grafted quartz fibres. [1]Romanov et al., Comp. Sci. Technol., 2015, 114, 79 [2]Qian et al., J. Mater. Chem., 2010, 20, 4751 [3]Qian et al., Comp. Sci. Technol., 2010, 70, 393