Cassava straw/polypropylene carbonate（PPC） composites samples were made at laboratory. Orthogonal array testing method with two factors and four levels was used to investigate effects of the calcium carbonates as a filler. Mechanical properties and thermal stability of the composites were evaluated. The results showed that mechanical properties of the composite samples filled with both nano-calcium carbonate and active calcium carbonate were better than that filled with heavy or light calcium carbonate. Particularly, bending strength of the composite samples filled with 15% nano-calcium carbonate presented 42.1 MPa, which exceeded the requirements（≥20 MPa） of China national standard GB/T24137—2009. Thermal gravimetric analysis（TGA） results further showed that thermal stability of the composite samples was improved to some extent by filling with calcium carbonates.