Abstract: To determine the standard Brinell hardness values of 10Cr9Mo1VNbN steel large diameter tee forgings, the statistic analysis on the Brinell hardness and tensile strength of a number of the same material forgings was performed and the formula of the relationship between these two characteristic parameters was given. The surface hardness test was done for the F91 (namely 10Cr9Mo1VNbN) steel large diameter forged tees used in 600 MW super-critical units at erection site to verify the hardness evenness. On the basis of the statistic analysis, testing results and requirements of the domestic and foreign standards for similar or same materials, it was determined that the standard Brinell hardness values of 10Cr9Mo1VNbN steel large diameter tee forgings should be controlled in 190-248 HBW and the hardness difference of one piece should be limited to no more than 40 HBW.