The invention relates to a method for determining the rod diameter of a rod mill, belonging to the field of ore grinding and levigating technology. The method provided by the invention comprises the following concrete steps: (1) measuring the uniaxial compressive strength alpha of the ore to be milled, as a mechanical basis for determining the accurate rod diameter; (2) performing screen analysis of particle size for the feeding material of the rod mill, and determining the fraction d'f and average particle size for ensuring the screening rate of 95%; (3) converting the practical working conditions of the rod mill into the working conditions of a ball mill in the same conditions, and calculating the ball diameter in the conditions by using a half-theory formula of ball size (C.G.S system); (4) correcting the calculated ball diameter according to the breaking experience and ore milling characteristics of the steel rod, and calculating the required rod diameter through the formula D(R-opt)=(0.48-0.5)D'b; and (5) performing ore milling contrast test with the rod diameter greater than and less than D(R-opt), and determining the accurate rod diameter of the rod mill according to the ore milling result. Compared with the prior art, the method is simple and convenient, is accurate in calculation result, has low labor intensity of workers and shortens the test period.