Polymeric composites reinforced with natural fibres have been developed in recent years, showing significant potential for various engineering applications due to their intrinsic sustainability, low cost, low weight and mechanical strength. The interfacial adhesion between natural fibres and polymeric matrices is critical to the composite performance. In order to improve the physical adhesion of polymeric composites, micro and nanoparticles have been added to synthetic fibres in the past. This work investigates the effect of silica microparticles, volume fraction of sisal and maleic anhydride on the mechanical properties of polymeric composites reinforced with unidirectional sisal natural fibres. A full factorial design (2231) was carried out to identify the effect of these factors on the responses: bulk density, apparent density, apparent porosity, water absorption, mechanical strength and modulus of elasticity. A microstructure analyses was conducted to verify the interface condition. The volume fraction of fibres, silica addition, and the interaction between silica particles and maleic anhydride additions exhibited significant effects on the tensile strength and modulus of elasticity of the composites. The microsilica addition did not affect significantly the flexural strength; while the interaction between fraction of fibres, silica particles and maleic anhydride addition played a major role not only on the flexural strength, but also on the flexural modulus. The volume fraction of sisal fibres exhibited significant effects on the bulk density, apparent density, apparent porosity and water absorption of the composites.