Abstract: The average intercept of grains of a metal was measured by straight-line cutting point method, each uncertainty component during measurement was evaluated, and then according to the uncertainty propagation law, the combined standard uncertainty and the final expanded uncertainty of the average grain size were obtained. The results show that under the test condition, the expanded uncertainty for measurement of average grain size of the metal U=0.1, when the coverage factor k=2, and the average grain size grade G=8.4±0.1. The smaller measurement uncertainty of this evaluation was related to the uniform grain size, clear grain boundaries, good condition of instruments, close measurement level of staffs, high accuracy of straight-line cutting point method and so on. During the measurement, attentions should be paid to above factors.