Poly (lactic acid) (PLA) bioplastics are recyclable and biodegradable thermoplastics. They are derived from environmentally friendly sources such as potatoes, cornstarch and sugarcane. However, PLA is inherently brittle with low impact strength. The goal of this study is to improve mechanical properties of PLA by the addition of calcium carbonate (CaCO3) fillers. PLA composites were prepared by injection molding conventional limestone (LS) and white chicken eggshell (WES) powders with particle sizes of 63 μm and 32 μm in amounts of 5 wt. %, 10 wt. % and 20 wt. %. Mechanical properties such as, tensile strength, tensile modulus, and Charpy impact strengths were investigated. These three properties were evaluated and the results statistically analyzed using ANOVA F-test. For both particle sizes, the tensile strength decreased as the filler content increased, but was highest for a filler loading of 5 wt. %. In general, the 32 μm powder fillers had better tensile strengths than 63 μm sized fillers. The tensile modulus increased with filler content and was highest at 20 wt. % for both particle sizes. The LS/PLA composites had better toughness than the WES/PLA composites. The particle filler morphology and fractured surfaces were observed by scanning electron microscopy (SEM) and determined to have well dispersed particles with smooth fractured surfaces. Water absorption behavior of PLA/CaCO3 composites were studied by immersion in distilled water at room temperature for 56 days. Virgin PLA absorbed the least amount of water while the water absorption of CaCO3 composites were a function of powder type and content.