This study aims to evaluate the influence of cyclohexane/ethanol, alkali, and silane treatments on the waste hemp fibers and their polybenzoxazine composites. The Fourier transform infrared (FTIR), scanning electron microscope (SEM), and thermogravimetric analysis (TGA) tests were carried out to understand the influence of treatments on the fibers. FTIR results revealed that hemicellulose and lignin of the fibers were removed after washing with cyclohexane/ethanol, and thermal stability of the fibers was improved after the alkali and silane treatments. The consequences of the treatments on the hemp fiber composites were studied by evaluating the mechanical, thermomechanical, thermal, and water uptake properties. The flexural, tensile and impact tests results confirmed that the silane treated fiber composites displayed highest properties than the other treated fiber composites. Furthermore, the stiffness and glass transition temperature of the silane treated fiber composites were also best among the studied composites. Moreover, the thermal stabilities values showed the lowest decline for the silane treated composites; due to the better thermal stability of the silane treated fibers, and lowest water absorption was also recorded for the silane treated fiber composites. The morphological studies confirmed that the silane treated fiber composite had stronger fiber/matrix adhesion interface.