Abstract: The hardness of base metal and weld of T91 steel small diameter tube with different wall thickness was tested by Leeb hardness tester and Brinell hardness tester respectively, and the effect of wall thickness on conversion relationship between leeb hardness and brinell hardness of T91 steel was studied. The results show that the error between the direct reading value and the measured value of the base metal decreased with the increase of the wall thickness. The error between the direct reading value and the measured value of the weld increased first and then decreased with the increase of the wall thickness, and finally tended to be stable. Based on regression analysis, a new conversion relationship between Leeb hardness and Brinell hardness of T91 steel small diameter thin-walled tube base metal and weld with different wall thickness was established. The absolute error between the calculated value and the measured value was 0-6 HB.