Anshan Benxi area in Liaoning province contains thelargest iron ore cluster in China, and many large superlarge iron deposits, including Gongchangling No.2 mine large scale high grade iron ore deposit, Qidashan and Nanfen middle scale high grade iron ore deposit. The low grade iron orebodies, metasedimentary and porodite occur as lenticular or lentoid in Archean granite. While the high grade iron orebodies occur as stratoid or lentoid in the low grade iron orebodies, wall rock and their fault zone. In order to discuss the genesis of the high grade iron ore in Anshan Benxi area, and provide more information for high grade iron ore prospecting work, this paper is concentrated on the comparative studies of the fluid inclusions, sulfur isotope, oxygen isotope and hydrogen isotope between the high grade iron ore and the low grade iron ore. The quartz in the low grade iron ore (include magnetite quartzite and martite) contains much gas inclusions (Ⅰc type inclusions), which is negative crystal. While the quartz in the high grade iron ore mainly contain liquid inclusions (Ⅰb type inclusions) and some daughter mineral bearing fluid inclusions. Pyrite δ34S of the low grade iron ore range from -6.5‰ to 11.8‰ with the average of 0.4‰.Pyrite δ34 S of the high grade iron ore range from -7‰ to 14.4‰, with the average of 2.0‰. In the low grade iron ores, the δ18O of magnetite range from 3.4‰ to 10.4‰ with the average of 7.1‰, the δ18O of quartz range from 11.8‰ to 15.1‰ with the average of 13.4‰; But in the high grade iron ores, the δ18O of magnetite range from -1.4‰ to 6.5‰ with the average of 2.3‰, the δ18O of quartz range from 9.6‰ to 15.8‰ with the average of 13.6‰, the δD of quartz range from -129‰ to -75‰ with the average of -104‰. The characteristics of fluid inclusions, pyrite δ34S and δ18 O of magnetite in the high grade iron ores are not only inherit the low grade iron ore, but also show characteristics of hydrothermal fluids. All of these suggest that the high grade iron ore may be formed by the low grade iron ore though activities of hydrothermal fluids. The analysises ofδD andδO show that the hydrothermal fluids fluid is mainly migmatization hydrothermal fluids. The high grade iron ore formed by the hydrothermal replacement of the low grade iron ore, not only through removal of silica but possibly also iron activation and transition model.