Abstract: The present invention relates to a novel crystalline disodium pamidronate 3˜4 hydrate and a process for preparing it. More particularly, crystalline disodium pamidronate 3˜4 hydrate of the following formula I having the water content of 17.0˜20.0 wt % and being in the state of the mixture of trihydrate and tetrahydrate is prepared by neutralizing pamidronic acid with sodium compound, drying the formed disodium pamidronate, dissolving the disodium pamidronate in water, filtering sparingly soluble materials, allowing the filtrate to stand with stirring at normal temperature to form crystals, cooling and filtering the mixture to obtain crystals, and drying at the temperature of 75˜85° C. under normal pressure. The novel disodium pamidronate 3˜4 hydrate according to the present invention is much more stable compound structurally than the known disodium pamidronate pentahydrate.