To reduce autogenous deformations in high performance concrete at early ages, dispersed saturated lightweight aggregates (LWAs) are used as water reservoirs. For this purpose, in a concrete with low water cement ratio, normal aggregates have been replaced by natural LWAs at size fractions of 2-4mm or 4-8mm at three different volume fractions such as 10, 20 and 30 percent of total aggregate volume of concrete prepared at a constant low water/cement ratio. Effects of volume fraction and average particle size of LWAs on the load-displacement at mid-span curve are investigated by measuring the fracture energy, the characteristic length and final displacement. The results indicate that the inclusion of fine fraction of LWAs in the concrete reduces the autogenous deformation significantly compared to that of the coarse LWA fraction. It is also shown that the fracture energy, final displacement at the mid-span curve of the beam, splitting tensile strength and compressive strength of concrete with fine fraction are higher than those of the concrete with coarse fraction. Increasing the replacement ratio of LWAs mitigates autogenous deformation, while having an unfavorable effect on fracture and mechanical properties of concrete for both fine and coarse fraction replacements.