This article relates to the melt-processing and properties of polylactic acid (PLA) reinforced with cellulose microfibrils (CMFs). The CMFs with diameters of 20–80 nm and lengths in the order of microns were isolated from wood sawdust. The purpose of the present study was to find a simple method to overcome the problems associated with feeding and aggregation of the nanoscale fibers in PLA melt. Two fiber carriers were compared, that is, natural rubber (NR) latex and polyethylene glycol (PEG) with a molecular weight of 4000 g mol−1. The results showed that with the aid of carrier, CMFs were successfully dispersed in the composites, enabling the strong reinforcing action of the fibrils to be realized. The type of carriers used had significant effects on the final properties of the PLA composites. Dynamic mechanical analysis results showed an eightfold improvement in modulus at elevated temperature (90°C) for the composite with 3 wt% CMFs using PEG as carrier. This enhancement was attributed to the combined effects of fiber reinforcement and cold crystallization induced in the PLA. With NR latex as carrier, the composite of high tensile strength was achieved by introducing the epoxidized rubber (ENR) in a ratio of 2:3 (ENR:NR) as a compatibilizer to improve adhesion between phases in the composites.