The effect of magnetic water (MW) on mechanical properties (MP) of lightweight concretes (LWC's) made with expanded polystyrene was investigated. Eight different types of water were used. The water used were drinking water and raw water of the Karun River that each was magnetised at 0.5, 1 and 2 Tesla fields. Slump, water absorption, static elasticity modulus (Ec) and compressive, tensile, and flexural strength tests were performed. The results showed that the use of drinking water compared to river raw water provides higher MP. Using drinking water compared to the raw river water increased the efficiency of concrete by 3% and the compressive, tensile, flexural strength and Ec by 10%, 3%, 3%, and 2%, respectively. Using MW up to the 1 Tesla field intensity has a positive effect on concrete MP. Due to the use of MW with 1 Tesla intensity, compressive, tensile, and flexural strength and Ec increased by 24.4%, 6.1%, 3.9% and 0.5% in concretes made with drinking water and 12.4%, 3.6%, 3.4% and 2% in river raw water concretes. However, the MP of LWC's containing MW with intensity of more than 1 Tesla were almost similar to the case in which MW with 1 Tesla was used.