In this research, an attempt was made to investigate effects of expanded perlite aggregate grain size on consistency, density, compressive strength, thermal conductivity and microstructure of 15 different composite mixes with silica aerogel. As for the samples preparation, expanded perlite aggregate of 5 different groups based on grain size, were used for sample preparation, then partially replaced by volume for 20% and 40% of hydrophobic silica aerogel particles. The results showed, that density of the samples varied between 0.35 g/cm3 and 1.5 g/cm3, flexural strength varied between 3.4 MPa and 7.4 MPa, compressive strength was in the range between 12.3 MPa and 55 MPa, thermal conductivity coefficient was in the range between 0.130 W/mK and 0.190 W/mK. Scanning electron microscopy showed that expanded perlite aggregates and silica aerogel particles are capable of being mixed and formed homogenous mixture. Nevertheless, microscope images indicated weaker adhesion of silica aerogel particles at interfacial zone as compared with expanded perlite aggregate particles. Results revealed, that both of the factors: grain size of expanded perlite aggregate particles silica aerogel content influenced the density, compressive strength and thermal conductivity. The study also indicated feasibility of expanded perlite aggregate and silica aerogel for achieving homogeneous mixture of the lightweight cementitious composites. Study demonstrated that using different size fractions of expanded perlite aggregate affects differently physical, mechanical and thermal characteristics of the lightweight cementitious composite with silica aerogel.