To investigate the effects of water binder ratio and coarse aggregate volume fraction on internal relative humidity(IRH) and moisture diffusion coefficient of concrete, development of IRH of a series of three concretes with water binder ratio of 062, 043 and 030 respectively, labeled as C3, C5 and C8 accordingly, was measured during early ages, and the moisture diffusion coefficient was obtained as well. The results show that with increased water binder ratio, the duration of early age moisture saturated stage becomes longer, self desiccation induced reduction of IRH decreases, moisture diffusion induced reduction of IRH increases, and the moisture diffusion coefficient becomes larger. Due to self desiccation only, the average IRH of sealed specimens of series concrete C3, C5 and C8 at 28 days are 954%, 933% and 796% respectively, while the corresponding IRH of the dried specimens under the combined effects of self desiccation and moisture diffusion are 700%, 732% and 730% respectively. The maximum moisture diffusion coefficient(Dmax) of ordinary concrete C3 is 10 times of that of high strength concrete C8. Furthermore, the moisture diffusion process inside early age concrete probably mainly depends on the mortar phase, regardless of the limestone aggregate. Merely increasing the volume fraction of limestone aggregate, within the percentage from 21% to 500%, the variation of IRH and moisture diffusion coefficient is negligible for concretes with the same water binder ratio.