This study evaluates the mechanical and thermal properties of polypropylene (PP) composites reinforced with sodium hydroxide (NaOH)-treated sisal fibers. The composites were fabricated using the hand layup technique, with sisal fibers ranging from 0–20 wt% (in 5 wt% increments). The sisal fibers were treated with 5% NaOH to enhance their adhesion to the PP matrix. The mechanical properties of the composites were tested according to ASTM standards, including tensile strength, flexural strength, and hardness. The results showed that the composite with 20 wt% NaOH-treated sisal fibers exhibited superior mechanical properties compared to the unreinforced PP matrix, with tensile strength of 72.5 ± 0.5 MPa, flexural strength of 103.5 ± 0.8 MPa, and hardness of 43 ± 0.6 HV. The study highlights the potential of NaOH-treated sisal fibers as a reinforcement material for PP composites, offering improved mechanical and thermal performance. The results suggest that the NaOH treatment enhances the adhesion between the fiber and matrix, leading to better mechanical properties. The study also emphasizes the importance of surface treatment in improving the performance of natural fiber-reinforced composites. The findings indicate that the use of NaOH-treated sisal fibers in PP composites can lead to better mechanical and thermal properties, making them suitable for various engineering applications. The study provides a comprehensive evaluation of the mechanical and thermal properties of PP composites reinforced with NaOH-treated sisal fibers, demonstrating their potential for use in various engineering applications.This study evaluates the mechanical and thermal properties of polypropylene (PP) composites reinforced with sodium hydroxide (NaOH)-treated sisal fibers. The composites were fabricated using the hand layup technique, with sisal fibers ranging from 0–20 wt% (in 5 wt% increments). The sisal fibers were treated with 5% NaOH to enhance their adhesion to the PP matrix. The mechanical properties of the composites were tested according to ASTM standards, including tensile strength, flexural strength, and hardness. The results showed that the composite with 20 wt% NaOH-treated sisal fibers exhibited superior mechanical properties compared to the unreinforced PP matrix, with tensile strength of 72.5 ± 0.5 MPa, flexural strength of 103.5 ± 0.8 MPa, and hardness of 43 ± 0.6 HV. The study highlights the potential of NaOH-treated sisal fibers as a reinforcement material for PP composites, offering improved mechanical and thermal performance. The results suggest that the NaOH treatment enhances the adhesion between the fiber and matrix, leading to better mechanical properties. The study also emphasizes the importance of surface treatment in improving the performance of natural fiber-reinforced composites. The findings indicate that the use of NaOH-treated sisal fibers in PP composites can lead to better mechanical and thermal properties, making them suitable for various engineering applications. The study provides a comprehensive evaluation of the mechanical and thermal properties of PP composites reinforced with NaOH-treated sisal fibers, demonstrating their potential for use in various engineering applications.