Digital image processing was used to measure the characteristics of particle group of lightweight aggregate,and multiple parameters were adapted to characterize the characteristic sizes.The effect of characteristics of particle group of lightweight aggregates on mechanical properties of lightweight concrete was then analyzed with grey correlation analysis method.Lightweight concrete of both high compressive strength and high flexural strength could be made with lightweight aggregates of specific surface area higher than 520 m2·m-3 and aspect of particle group lower than 1.34 as well.The results provide a scientific basis for choosing lightweight aggregates when making lightweight concrete.