Model composites with Polypropylene (PP) as matrix and alfa fibres (cellulose-based fibres obtained from the esparto grass of the subsaarian dry regions of Tunisia) were prepared for fibre/matrix interfacial characterization studies. The matrices tested were PP and PP combined with PP modified with maleic anhydride (PP-g-MA). The surface of the alfa fibres was treated by air plasma treatment. The adhesion between the untreated and treated fibres and both matrices was studied using the fragmentation test method. Composites with 10% weight of fibres were prepared by melt extrusion and injection moulding, and the specimens obtained tested for tensile properties. The fracture surfaces of the composites, obtained at low temperature, were observed by scanning electron microscopy. The presence of a small concentration of maleic anhydride grafted to the polymer matrix was found to be of the utmost importance for the establishment of a good fibre/matrix interface. The air plasma treatment had a cleaning effect of the fibre’s surface.