The mechanical properties of a solid ankle-foot orthosis (SAFO) are essential and can directly affect its capability of restricting ankle joint and impact the gait patterns in a population with movement disorders. A better understanding of the effects of materials, reinforcement techniques, and annealing on its mechanical properties will be useful and helpful for practitioners to improve the criteria of both design and prescription. The objective of this preliminary study was to provide quantitative outcomes regarding the mechanical properties of a SAFO with various materials, reinforcement techniques, and heat treatment. Eight SAFOs were fabricated with three types of thermoplastic materials (homopolymer polypropylene, copolymer polypropylene, and carbon-infused homopolymer polypropylene) and two reinforcement methods (corrugated and compcore). In addition, the homopolymer polypropylene SAFO without reinforcement was annealed at 185°F. A computer-controlled motorized test apparatus was used to load the SAFOs within 20 Nm resistance torques in both dorsiflexion and plantarflexion at 5°/s. The mechanical properties of the SAFOs, including range of motion, quasi-static stiffness at selected resistance torques, and index of hysteresis, were quantified based on the loading curves. Our preliminary results show that materials, reinforcement techniques, and heat treatment affect the mechanical properties of the fabricated SAFOs to different levels. Among the materials used, carbon-infused homopolymer polypropylene outperforms the other two popularly used polypropylenes. The compcore reinforcement technique is more favored than the corrugated reinforcement technique. Lastly, annealing has shown promising effects on improving the overall mechanical properties of homopolymer polypropylene SAFOs.