The aim of this work was to study the effects of alkali and alkali-silane surface treatments on Spartium junceum (Sj) fibers characteristics. Then, untreated and surface treated fibers were incorporated into a poly(lactic acid) matrix and the properties of the composites were studied. To improve the adhesion at the interface between the PLA matrix and alkali-silane treated fibers, maleic anhydride-grafted poly (lactic acid) (PLA-g-MA) was added as a compatibilizer. FTIR and TGA analysis proved the efficiency of the alkali treatment in removing some non-cellulosic components, namely hemicelluloses and lignin. Also, XRD results showed that the applied chemical treatments contribute to the increase in the crystallinity index of the Sj fibers. The incorporation of Sj fibers was found to decrease notably the impact strength of PLA and to affect its thermal behavior by increasing the melting temperature of the composites. However, it has been observed that the treatments increased notably the composites impact properties. These results have been confirmed by scanning electron microscopy (SEM) observations of the composites fractured surfaces which revealed a more intimate contact between the Sj fibers and the PLA matrix after surface modification. Also, the PLA-g-MA acted positively on the adhesion in the interfacial region.