Addition of small amount of boron (B) in the (9~12)%Cr martensitic heat resistant steels can obviously prohibit the Ostwald ripening of M₂₃C₆ carbides so as to improve creep strength as well as creep rupture life. With the purpose of taking full advantages of B element, it is critical to make B preferentially distribute in (9~12)%Cr martensitic heat resistant steels. The mechanism of B preventing M₂₃C₆ carbides from ripening is also on the premise of clearly identifying the preferential distribution of B in the steels. Much concern has been growing over the preferential distribution of B in the research of (9~12)%Cr martensitic heat resistant steels. Therefore, this article gives a review on this aspect. Following a summary of the effect of B on mechanical properties, several commonly used characterizing methods for B segregation in the steels are introduced. Based on the physical metallurgy and the solution, diffusion mechanisms of B element, discussions on the preferential distribution of B element at prior austenite grain boundaries and in the M₂₃C₆ carbides as well as the related factors are emphasized. At last, two prevalent mechanisms of B restraining the coarsening of M₂₃C₆ carbides in (9~12)%Cr martensitic heat resistant steels are given by an intensive explanation so that the relationship between the preferential distribution of B and its advantage of increasing creep performance by suppressing the ripening M₂₃C₆ carbides are systematically elaborated, which gives a deep understanding of the role of B element in (9~12)%Cr martensitic heat resistant steels.