The purpose of this study is firstly to investigate the effect of various treatments on the chemical and morphological properties of the fibers of two xerophyte plants (Doum Chamaerops humilis' and Diss Ampelodesmos mauritanicus'), in order to improve the adhesion between fibers and cement matrix, secondly the effect of increasing inclusion rates of these two fibers on the physical, mechanical and thermal properties of cementitious composites. Obtainable results by infrared spectroscopy, X-ray diffraction and scanning electron microscopy revealed that the alkaline treatments of the Doum and Diss fibers with 1 and 3% NaOH for 30min at room temperature resulted in an optimal improvement of the crystallinity index and alter the characteristics of the surface topography. The reinforcement of mortars with these two fibers caused a decrease in their bulk density and consequently the compressive strength, as well as an increase in their porosity and thus their capacity of water absorption. Moreover, the mortars reinforced with 2% of treated Diss fibers and 1.5% of treated Doum fibers show the high flexural strengths. Also, thermal conductivity measurements were carried out for different drying durations to assess the effect of water content on the thermal properties. Thermogravimetric Analysis coupled with differential scanning calorimetry are also performed to study the degradation and mass loss of composites at high temperature. The results reveal that the incorporation of these fibers reduces the thermal conductivity and increases the heat absorption of the composites.