Fractal theory was adopted to measure the interfacial roughness of lightweight concrete. The roughness represented by fractal dimension and its relation to the mechanical properties of lightweight concrete was studied. The relation between the characteristics of particle group of lightweight aggregates and interfacial roughness fractal dimension was studied with grey correlation analysis method. Then the effect of characteristics of particle group of lightweight aggregates on interface properties of lightweight concrete was analyzed. The results show that the interfacial roughness shows great effect on the mechanical properties of lightweight concrete. The specific surface area and aspect of particle group have greater influence on the interfacial roughness of lightweight concrete. When the specific surface area of particle group is between 540m2/m3 to 630m2/m3 and aspect of particle group is 1.32 to 1.33, the interface and mechanical properties of lightweight concrete are better.