Presented is a new method of manufacturing PAC(polyaluminum chloride) used as flocculant for water treatments : PAC is obtained through partial decomposition of aluminum chloride hexahydrate at 180℃ to give basic aluminum chloride followed by dissolving the basic chloride in water. With varying the extent of decomposition, properties of the solid-state basic chloride and the PAC solution were analyzed. The IR analysis showed that at an extent of decomposition of 33.4% an absorption band appeared at 980cm^-1 indicating the formation of basic aluminum chloride. The basicity of the PAC solution was observed to increase with increasing the extent of decomposition. In order to meet the basicity of 45% required for commercial grade PAC, it was found necessary to have an extent of decomposition higher than 45%. According to the NMR analysis, both[Al(H₂O)₆]⁺³ monomer and aluminum chloride polymers were present in the PAC solution. The molar ratio of the polymers to the monomer increased with an increase in the extent of decomposition. The PAC manufactured by the present method was tested on the water taken from the Kum river.