This paper presents the results of experimental investigations on the behaviour of both high (HSC) and normal strength (NSC) concrete at early ages. The mechanical experiments included compression test, splitting tension, and fracture test. The tests were conducted at ages of 12 h, 1, 2, 3, 7 and 28 days. In addition to various strengths, the complete stress - strain curves of concretes under uniaxial compression at different ages were obtained by utilising the circumferential control method. It was found that the strength development rate of HSC was sharper than that of NSC at early ages. To investigate the microstructure development of concrete at early ages, mercury intrusion porosimetry (MIP) was used to examine the pore size and accumulated pore volume for both HSC and NSC at ages of 12 h, 1, 3 and 28 days. The results showed that the porosity decreased with the progress of hydration. Accordingly, the strengths of both HSC and NSC increased as the result of progress of microstructure development. © 2005 Thomas Telford Ltd.