Considering the fact that Irahosts a large number of earthquakeand is one of the most seismiccountriein the region therefore designing and building lightweight structures is of great importance. The weight reduction of a building, in addition to saving on cost, time and energy, reduces the damages caused by natural disasters such as earthquakes and minimizes the damages caused by the heavy weights of the building. In recent years, lightweight concrete using leeka particles has drawn much attention due to features such as low weight, sound insulation and resistance, and good resistance to heat and cold. Since the use of fibers in concrete structures can greatly enhance the strength and durability parameters of concrete, this thesis examines the effect of the elastic fiber modulus used in improving the mechanical properties of light concrete. For this purpose, three different modules of polypropylene fibers with low, medium and high elasticity in the range of 0.5-1 Gpa, 2.5-4 Gpa, and 4.5-6 Gpa, taking into account the various volumetric percentages of the fibers ( 0.25 and 0.5) and lengths of 6, 15 and 24 mm fibers, 20 mixing designs in the first phase of the study, and the slump tests, the density and dryness, the compressive strength, the tensile strength and flexural strength on the specimens. In the second phase of the study, the effect of Pososolanic materials of Microsilica and Nanosilica on the improvement of the mechanical parameters of the optimal mixing scheme of the first phase has been experimented. In this study, 385 cube, cylindrical and prismatic-shaped concrete samples have been tested. The results of the research show that the modulus of elasticity of the fibers consumed in the compressive strength of the lightweight concrete does not affect the tensile strength, but increasing the modulus of elasticity of the fibers can affect the tensile strength and flexural strength of the lightweight concrete composite specimens. The effect of the modulus of elasticity of the fiber on the properties of the flexural strength of the concrete is until the phenomenon of the cellularization of the fibers has not occurred. In this study, fibrous pellets were observed in some mixing designs containing 0.5% volumetric fibers with a length of 24 mm. The percentage increase in tensile strength for 6 mm long fibers for modulus of low, medium and high modulus of elasticity used in this study was 4, 9 and 16 for a volumetric percentage of 0.25 fibers and 13, 21 and 36 for a volumetric percentage of 0.5% 0 fiber is relatively lightweight non-fibrous concrete. Also, the greatest increase in tensile and flexural strength is observed for 15 mm long fiber-containing specimens. In less volume of 15 mm fiber (0.25%), the tensile strength is 11, 15 and 22% for low, medium and high modulus elasticity. Also, for a greater volume of 15 mm fibers (0.5%), the tensile strength increased by 15, 29 and 52%, respectively, compared to a non-fibrous concrete sample. Also, the results of the usage of Pozzolan Microsilicon at 10% cement weight replacement in the mixing design for the first phase of this study showed that the reduction of compressive strength resulting from the use of PP fibers in the concrete mixing scheme is also greatly compensated. The use of Nano silica in 1-3% of the microcrystalline substitute has led to a decrease in the strength of parameters of light fiber concrete. Keywords : Structural lightweight concrete, Leca, Polypropylene fiber, elasticity fiber modulus, Microsilica, Nanosilica