Pine cone and Spartium junceum are readily available cellulosic materials which can be used as reinforcements in thermoplastic-based composites. In the present study, flours issued from Pine cone (PCF) and Spartium junceum (SjF) were first chemically modified with maleic anhydride (MA) and silane. Then, neat and treated flours were incorporated into a polypropylene (PP) matrix and the properties of composites prepared without or with maleic anhydride-grafted-polypropylene (PP-g-MA), added as a compatibilizer, were compared. FTIR analysis proved the efficiency of the treatments in removing some non-cellulosic components and formation of ester linkages due to the reaction between MA and the flours hydroxyl groups. In the case of Silane-PCF, the bands observed around 1730 cm(-1) and 1241 cm(-1) disappeared, thus indicating that the silane treatment allowed the extraction of hemicellulose and lignin from the flour surface. X-rays diffraction results showed that the applied chemical treatments contributed in increasing the flours crystallinity index, which implies that by the removal of the amorphous material covering the fibers, the more crystalline cellulosic component of the fibers is exposed. The incorporation of PCF and SjF increased the composites crystallinity which emphasizes their significant role in heterogeneous nucleating of PP matrix. Additionally, the composites observation by electron microscopy revealed more intimate contact between the flours and PP matrix after surface modification which permitted the dissolution of impurities. As a result, treated PCF and SjF induced a significant increase in the composites resilience and decreased notably their water uptake aptitude relatively to the composites based on untreated flours.