Biocomposites made from renewable resources have a lower environmental impact than inorganic composites. However, these new biocomposites must have the same mechanical properties as traditional composites to be able to replace them. The main objective of this study is to analyse the influence of strain rate and fiber content on the mechanical behaviour of a new biocomposite composed of a biodegradable PolyHydroxyAlkanoate (PHA) matrix reinforced by Alfa fibers. These biocomposites were elaborated after an alkaline treatment and fiber bleaching procedure. SEM and chemical analysis showed that these treatments remove the non-cellulosic materials such as pectin, hemicellulose and lignin, and improve the cellulose content. Three fiber contents of 5, 10 and 15%wt and three strain rates were investigated to analyse their dependence on uniaxial tensile tests. By increasing the amount of fiber and the strain rate, the mechanical response was improved with higher Young's modulus and tensile strength but lower elongation at break. Poisson's ratio have also shown dependence on strain rate and fiber contents. These results underline the good mechanical properties of this PHA/Alfa biocomposite which could be an interesting alternative in the development of products, such as automotive interior parts or some packaging applications.