Recently a large number of concrete structures are build with poor quality and low dual ity concrete as a result of using much amount of water at the placing in a tight schedul of construction. Any effective method to evaluate the amount of water at the placing of the concrete, however, has not established yet. In order to invent a new method to evaluate the water cement ratio at the placing, we inves tigated the shape of the calcium hydroxide in the concrete and found that it grew into a large size of crystal when a much amount of water was used at placing. Based on the X-ray powder diffraction analysis of a series of concrete with different amount of water cement ratio, it was shown that the size of the calcium hydroxide was closely related to the intensity ratio of certain diffraction peaks and that resultingly there was a strong correlation between the water cement ratio and the size of calcium hydroxide. Calling the intensity ratio as "water index" we proposed a new method of evaluating the original water cement ratio.