In order to investigate how particle aggregation affects tensile mechanical performance of composite, a ductile biopolymer (poly(caprolactone)) was melt-compounded with CaCO3 particles over a wide concentration range from 10 to 60 wt.%. The aggregation of CaCO3 particles in poly(caprolactone) (PCL) is investigated depending on particle concentration and surface modification (with stearic acid (2.5 wt.%)) based on rheological assessment. If the composite is mixed with a high concentration of particles (> 30 wt.%), morphological observations and a thermal behavior analysis do not find a difference in the particle aggregation regardless of particle surface modification. However, the linear viscoelastic moduli of the composites distinguishes the difference in particle aggregation regarding to surface modification, indicating induced aggregation behavior with surface-modified CaCO3 (sCC). The composite with sCC starts to form network structure of particles at a lower concentration (30 wt.%) than that with unmodified particles (40 wt.%). When particles form the network structure above the particle percolation threshold, the yield strength of the composite begins to decrease even though Young’s modulus is still increasing. In contrast to the expectation of the better dispersion of particles by surface modification as well as improved tensile mechanical performance with better dispersion, sCC rather induced aggregation with a lower concentration of particle than unmodified particles which resulted in decrease in yielding performance. This study showed that rheological study, especially for the composite with high concentration of particles, is useful to figure out the particle dispersion against a limit at morphology observation.