In order to detect the influence of age on the experiment that use scanning electron microscopy (SEM) to determinate the original water-cement ratio (w/c) of hardened concrete, a series of concrete mixes are tested. Experimental results show that early ages do have an effect on the water-cement ratio of hardened concrete that determined from scanning electron microscopy, this effect will decrease while age increases. The relationship between capillary porosity, w/c and age is analyzed and a calculation equation is given.