Abstract: Inclusion distribution in 16.8 t ingots of high pressure boiler tube steel P12 was studied by dissection, total oxygen analysis, original position statistic distribution analysis (OPA), metallographic microscope static analysis and small sample electrolysis. It was found that a negative inclusion segregation area existed in the central part of the ingot head where the inclusion quantity is lower; while a positive inclusion segregation area existed in the center and lower part of the ingot where the inclusion quantity is higher. A new concept of inclusion segregation index was put forward to describe the segregation degree of inclusions. Total oxygen analysis and OPA results demonstrated that the segregation indexes of oxide inclusions were 1.4 to 1.6 in the central and lower part of the ingot and 0.5 to 0.7 in the central part of the ingot head. As both oxide and sulphide inclusions were tested by metallographic microscope static analysis and slime small sample electrolysis, the inclusion segregation indexes were about 0.7 to 0.8 in the central part of the ingot head and 1.15 to 1.35 in the center and lower part of the ingot. The average size of oxide inclusions in the central and lower part of the ingot was bigger than that in other places, showing that the main mechanism for inclusion segregation in the central and lower part of the ingot is the picking of large inclusions by showering crystallites during floatation.