Many researchers have rigorously studied the nonlinear behavior of the stress-strain relationship of concrete using mathematical curves. Currently, most empirical expressions for the stress-strain relationship, however, have been focused on hardened concrete and are unable to completely represent the behavior of concrete at an early age. A broad understanding of the behavior of concrete from early age to old age is important in evaluating the durability and service life of concrete structures. In this paper, the effect of five different strength levels and various ages from 12 It to 28 days on the 0compressive stress-strain curve was observed experimentally and analytically. Tests were carried out on phi 100 x 200 mm cylindrical specimens cured in a standard moist room at a temperature of 20 +/- 3 degreesC. An analytical expression of the stress-strain curve with strength and age was developed using regression analyses on experimental results. For verification of the proposed model equation, the equation was compared to the experimental data and existing model equations. The result shows that the proposed model equation is not only compatible with the experimental data but also describes satisfactorily the effect of strength and age on the stress-strain curve. (C) 2003 Elsevier Science Ltd. All rights reserved.