In this study, the effect of SiC particle size on the sintering behavior of SiC-Si nano composites fabricated by spark plasma sintering (SPS) technique was investigated and a model was proposed, accordingly. To this purpose, SiC powders with three different particle sizes of 25 µm, 80 nm and 45 nm were chosen. It was expected that hardness of the composites increase with decreasing the SiC particle size; however, the outcomes were interesting and unpredictable. The composite with 80 nm SiC particles indicated the highest hardness. Hardness of the specimen with 25 µm SiC was low because of the large particle size of its reinforcement. While 80 and 45 nm SiC particles are considered as nano particles, the composite with 45 nm SiC particles showed lower hardness due to the growth of SiC powders during sintering according to a proposed model. Two reasons for the growth of 45 nm SiC particles were defined: (i) the fineness of the SiC particles prevented the Si particles to act as a binder between them thus, they agglomerated; (ii) SiC powders were oxidized during mixing procedure and a layer of SiO2 was formed on their surfaces. During sintering procedure, the reaction between SiC and SiO2 was happened and as a result SiO was formed. It caused vapor transportation during sintering leading to necking between particles and in turn, grain growth.