The interfacial zone between cement paste and aggregate plays a critical role in determining mechanical properties of concrete. In this study, the effects of bond strength between aggregate and mortar, aggregate and cement paste on the mechanical properties of lightweight and ordinary concrete were examined. The flexural and compressive strengths of specimens of lightweight and ordinary mortars, the aggregate used in production of lightweight and ordinary concrete, and pure cement paste were experimentally determined together with the bond strength between aggregate and mortar, aggregate and cement paste. In experimental studies, natural lightweight aggregate and normal aggregate with Portland cement were used. Some basic expressions obtained from using aggregate strength, mortar strength, cement paste, and bond strength were given to estimate the compressive strength of lightweight and ordinary concretes. (C) 2003 Elsevier B.V. All rights reserved.