Comonomer effect on the mechanical and morphological behavior of the calcite (stearic acid coated calcium carb onate)-filled polypropylene (PP), poly(propylene-random-ethylene) copolymer (CoPP), and poly(propylene-co-ethylene-co-1-butene) terpolymer (TerPP) composites were investigated by using dumbbell bar and film specimens. The tensile properties of the calcite-filled PP, CoPP, and TerPP composites exhibited lower values than those of the pure polymers (calcite-unfilled polymers), whereas the complex viscosity of the calcite-filled polymers exhibited slightly higher values than that of the pure polymers. Mechanical properties studied by using various strain rates and draw ratios rationalized in terms of comonomer units and contents in various PP systems. Morphological behavior of the specimens stretched at various strain rates and draw ratios was investigated by using SEM microphotographs and the mechanism of the formation of air holes was proposed. The air hole initiated from crack propagation and followed by dewetting between the calcite surface and the polymer interface in the weakened region. The crack propagated along the transverse direction; then the air hole developed parallel to the machine direction with fibril structure of the resin in PP and CoPP systems. However, TerPP composite exhibited no cracks in the beginning of the elongation, but the air hole was initiated due to dewetting; then its enlargement was exhibited by broken fibril structure of the resin. In the final stage of stretching, the air hole was dominated by merging of the neighboring air holes. Thus, different comonomer units, which are the small content of ethylene and 1-butene in CoPP and TepPP, are responsible for these systems behaving in a different manner on the mechanical and morphological properties.