The physical, chemical and mineralogical characteristics of Kenaf fibers from the Republic of Benin were studied by using the X-ray diffraction (XRD), infrared spectroscopy (IR), thermal gravimetric analysis coupled with differential scanning calorimetry (TGA/DSC) and chemical analysis using the method of VAN Soest. It emerged fron study that the fibers of Kenaf essentially consisted of cellulose (73wt %), hemicellulose (18wt%) and lignin (6wt%). Kenaf fibers of 10, 20 and 30 mm lengthwere used at dose rate of 1.2% weight to reinforce the mechanical and thermal properties of Compressed Earth Blocks (CEB) developed from a clay soil; constitued of kaolinite (33, 46wt%); illite (14.90wt%%); microline (2.11wt%); quartz (48.78wt%) and 0.86wt% of anatase. The incorporation of Kenaf fibers permitted to reduce the distribution of cracks in CEB. Analysis of the mechanical behavior of different formulations soil/fiber in terms of flexural strength and compression demonstrated the beneficial effect of the fibers. The best result of mechanical strenght standpoint was obtained with the fibers lenght of 30 mm. Here we sucessfully proved that is possible to improve the mechanical and thermal properties of CEB by using fibers for reinforcement; and the composite can be optimized by altering the fiber content and length.