Abstract: Natural fiber, as an alternative to synthetic fiber, is of great potential to reinforce composites that are applied in engineering fields such as automotive aerospace, automotive, sports, packaging, medical, and construction due to their renewability, environmental friendliness, high specific strength, and modulus. To realize this potential, ramie fiber reinforced poly (lactic acid) (PLA) composites with different fiber loadings were fabricated by injection molding. The heat deformation temperature, microstructure, crystallization behavior, rheological behavior, and mechanical properties of the composites were also analyzed. Results indicated that the heat resistance of the composites was improved with increased fiber loading. Particularly, the heat deformation temperature of the composites was improved by 10.5% when fiber with mass fraction of 40% was blended into the matrix. In addition, there were numerous fiber pull-outs and holes in the fractured surface due to poor interfacial adhesion between the fibers and PLA. Meanwhile, ramie fibers were uniformly distributed in the matrix when incorporating a low fiber content, but fiber agglomerations occurred in the matrix when introducing a high fiber loading (mass fraction of 40%) because of the poor wettability between the fibers and PLA. Differential scanning calorimetry (DSC) showed that the high fiber loading in the composites restricted the movement of the PLA molecular chain and promoted the formation of the perfect crystal. At the same time, samples with a high content of fiber contributed to the enhancement of the storage modulus, loss modulus, and complex viscosity of the composites due to the fibers’ physical joint in the matrix. Finally, the tensile and flexural strengths of the composites were improved with increased fiber loading. However, when the mass fraction of loading fiber was greater than 30%, the increase of tensile and flexural strengths of the composites was slow due to the weak wettability of the PLA matrix to the fiber. Compared to PLA, the incorporation of fiber with mass fraction of 40% increased the tensile and bending strengths of the composites by 30% and 21.9%, respectively.