In this paper, carbon particles with micro- and nano- particle size were synthesized through a hydrothermal reaction of glucose, namely C-1(123.1 nm), C-2(229.2 nm), C-3(335.1 nm), C-4(456.2 nm) and C-5(534.0 nm) with distinct sizes. We utilized five size carbon particles as individual fillers into the EHS matrix materials to prepare composite eutectic phase change materials (C/EHS PCMs) by melt blending technique. The impact of carbon particle size on the dispersion stability and thermal properties of Na2SO4·10H2O-Na2HPO4·12H2O (EHS) phase change materials was investigated. It is shown that adding 0.2wt% C-2 can decrease the supercooling degree of EHS to 1.5 C. The cyclic stability of C/EHS varies significantly depending on the size of carbon particles. The results of differential scanning calorimetry reveal that the incorporation of C-1, C-2, C-3, and C-4 into EHS lead to an enhancement of latent heat. The latent heat capacity of EHS with 0.2wt% C-2 is 243.4 J·g-1, and after undergoing 500 cycles of solid-liquid phase transition, the latent heat remained above 200 J·g-1. The C-2/EHS composite phase change material holds significant potential for advancing building insulation and solar energy storage technologies.