The aim of the current study is to further investigate the impact of the sisal fiber content up to 30% on the mechanical and thermal properties of polylactic acid (PLA) matrix composites following up to our previous study for sisal content up to 15%. The biocomposites were prepared successfully using a twin-screw extruder and an injection molding machine. It was found that 20 m% resulted the best mechanical, dynamic mechanical, and thermal properties among the tested fiber contents. Beyond this weight fraction of sisal, these properties showed a slight decrease probably because of reaching the threshold percolation corresponding to weight fraction limit before the aggregations started to form in the matrix. This finding was supported by scanning electron microscopy. The degree of crystallinity increased until reaching 10% fiber content, but not above. The results reveal that the reinforcement of PLA by sisal fiber does not only improve mechanical properties but also it could be used as a nucleating agent for the PLA. Moreover, a new relationship is proposed to characterize the effectiveness of the composite due to the addition of sisal fiber. The performance of the biocomposites developed makes them capable for applications in various fields such as aeronautics, automotive, and construction.