The aim of this study was to enhance the mechanical properties of milkweed floss (MW)/polylactic acid (PLA) composites by compatibilizing the interface between MW and PLA. MW floss is a natural fiber exhibiting a wide lumen and a thin cross-section. The addition of high specific surface area MW fibers to polymer matrices can lead to significant improvements of composites' mechanical performance. However, MW floss presents a poor bonding with PLA. This study evaluates the improvement of the adhesion and, consequently, of the composites' mechanical properties after MW modification with alkaline treatment or grafting of epoxy-silane. First, the influence of both treatments on the geometry, mechanical properties and roughness of MW fibers was investigated. Injection-molded composites made of 1wt.% treated MW were then characterized. Results showed that the mechanical properties of treated MW significantly decreased. Diminutions of 68 and 60% of Young's modulus (E) and ultimate tensile stress (UTS) were respectively measured for alkali treated MW (NaOH-MW). Reductions of 30 and 60% were respectively calculated for E and UTS of epoxy-silane treated MW (Si-MW). Still, improvements in the composites' mechanical properties were measured, such as increases of the UTS of 9 and 17% for PLA reinforced with NaOH-MW and Si-MW, respectively.