Lyocell fabrics were treated with 3-glycidoxypropyltrimethoxy silane (GPS) at different concentrations of 0.5, 1, and 3 wt%. Lyocell/PLA composites were then fabricated by a compression molding process using a film-stacking manner, which was stacking poly(lactic acid) (PLA) films between the lyocell fabrics, and their mechanical and thermal properties were explored. The tensile and flexural properties, heat deflection temperature, and thermo-dimensional stability of PLA were largely increased due to a reinforcing effect by the lyocell fabrics. The properties were further increased by silane treatment done to the lyocell fabrics prior to composite fabrication, showing the highest improvement with 1 wt% GPS. The effect of silane treatment on the improvement of the mechanical and thermal properties was consistent with the effect of silane treatment on the interfacial bonding between the lyocell fiber and the PLA matrix of the composite as well as on the lyocell single fiber tensile properties.