Natural rubber (NR), an important natural polymer derived from the Hevea brasiliensis tree, has been widely used in the rubber industry owing to its excellent elastic properties. However, it requires reinforcing fillers to improve its mechanical properties for the manufacturing of rubber products. Generally, calcium carbonate (CaCOsub.3) is employed as a non-reinforcing filler. This work aimed to synthesize spherical-shaped CaCOsub.3 at a submicrometric scale without and with surface treatment and explore its utilization as a reinforcing filler in NR composites. The morphological shape and polymorphic phase of CaCOsub.3 were investigated using SEM, TEM, XRD, ATR-FTIR and Raman techniques. The mechanical properties of various amounts (0 to 60 phr) of CaCOsub.3-filled NR composites were explored. As a result, the NR/treated CaCOsub.3 composites provided higher tensile strength than the NR/untreated CaCOsub.3 composites and pure NR at all filler loadings. This may have been due to the improved interfacial interaction between NR and CaCOsub.3 with the improved hydrophobicity of CaCOsub.3 after treatment with olive soap. The optimal filler loading was 20 phr for the highest tensile strength of the rubber composites. In addition, the elongation at break of the NR/treated CaCOsub.3 was slightly decreased. Evidence from SEM and FTIR revealed the vaterite polymorph and shape stability of CaCOsub.3 particles in the NR matrix. The results demonstrate that the particle size and surface treatment of the filler have essential effects on the mechanical property enhancement of the rubber composites. Synthesized spherical CaCOsub.3 could be a potential reinforcing filler with broader application in polymer composites.