Abstract: The delamination flaws were observed at the end of P110 steel grade oil casing pipes after heat treatment. Means such as chemical compositions analysis, metallographic examination, and SEM and EDS analysis were used to analyze the delamination flaws forming reasons. The results show that the delamination flaws were intergranular cracks generated during hot rolling, and interdentritic manganese segregation was the main reason for the delamination flaws.