In recent years the utilization of natural fibers as reinforcing agents in thermoplastic composites has captured major attention. This research studied the effect of reaction time on the morphology, particle size, and melting point of microcrystalline cellulose and the reinforcement ability via mechanical characterization of the modified MCC in a blend of predominantly PLA matrix. In order to accomplish this latter goal, the acetylated microcrystals were blended with polylactic acid (PLA) polymer at 5 wt% concentration. Furthermore, a new method to control the degree of substitution and particle shape, size and distribution of microcrystalline cellulose by controlling the acetylation time is presented. A series of samples were prepared by the acetylation of microcrystalline cellulose to varying levels of acetate substitution. Composites of the acetate substituted microcrystalline cellulose with polylactic acid (PLA) showed a substantial increase in the modulus of rigidity at an optimum concentration and level of acetate substitution. Spectroscopic, morphological together with thermal and mechanical property data are presented and discussed.