Numerous researches have paid attention to biocomposites. Biocomposite material can be defined as a composite material obtained by products derived from renewable resources such as polylactide acid. The aim of this work was to study biodegradable composites prepared by using cellulose microfibrils (CMFs) as the reinforcement and poly (d-lactic) acid as a matrix. Five average fiber lengths, from 8 to 300m, were used for the biocomposite preparation. The thermomechanical properties, crystallization, and the composites morphology were characterized by the means of dynamic mechanical thermal analysis (DMTA), tensile test device, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy. The effect of fiber length on the thermomechanical properties and crystallinity rate of the composites was investigated. The DMTA results showed that the storage modulus increases with the addition of CMFs. The X-ray diffraction studies, performed on the biocomposites, showed that the crystallinity rate of the blends was improved.