To seek an appropriate additive for the preparation of erythritol-based composites, the evaluation and comparison of composites with the addition of EG (expanded graphite) and CNTs (carbon nanotubes) have been conducted in this paper. Composites with additive mass ratios of 1 wt%, 3 wt%, 5 wt% and 7 wt% were prepared by melting dispersion. The thermophysical performances of the composites were discussed in terms of melting point, latent heat and thermal conductivity, which were characterized using Fourier transformed infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and the transient hot wire (THW) method. The cost effectiveness of the composites was also considered from the point of view of two indexes, i.e., thermal conductivity per unit cost k(c) and heat capacity per unit cost Q(c) The results revealed that the melting point of composites with EG continuously decreased with increasing mass ratio of additive due to the surface energy variation, while for the CNTs composites, it remained nearly constant. The latent heat of both composites gradually decreased as a function of mass ratio because of the replacement of erythritol by additives. The thermal conductivities of the composites also increased continuously with increasing addition of EG/CNTs. At the same mass ratio, EG appeared more effective than CNTs in enhancing the thermal conductivity, especially above 3 wt%. The optimal proportion of EG for the erythritol-based composite, with respect to not only the variation of thermal conductivity but also the heat capacity and cost effectiveness, was approximately 4 wt%.