The characteristics of optimum seed crystals for phosphorus removal have been studied comparatively by a column reactor for the leaching test and the phosphorus removal test by crystallization. As seed materials, phosphorus rock, electron arc furnace slag, blast furnace slag and converter slag have been chosen. The converter slag shows the most leaching quantity of Ca^(2+) and OH ion among the above slags. The average phosphorus removal efficiencies of phosphorus rock, electron arc furnace slag, blast furnace slag and converter slag are 35.9%, 74.2%, 99.1%, 94.5% under 20, l7, 24 and 158 days water flowing condition, respectively. X-ray diffraction pattern shows that blast furnace slag and converter slag transform from amorphous to crystalline phase. The SEM observation reveals that the finely distributed cubical crystalline are formed on the surface of converter slag during the crystallization period. The composition ratio (=Ca/P) of crystalline analyzed by EDS mapping is found to be 1.97 which is determined to be hydroxyapatite. By comparing the 4 seed crystals in technical and economical aspects, it is concluded that the converter slag is the best seed material for phosphorus crystallization process.