Nano-grained WC-10Co hardmetal can be made by doping a new invented VC-based multi-grain-growth-inhibitor and suitable technical processing. The specifications for quantitative measurement of the grain size of WC in the alloy were investigated. It is very important to obtain the SE images of the alloy with good contrast and clear profile of WC grains by FESEM; then the linear intercept (LI) method was used to quantitatively measure the intercepts of WC grains. When the surveyed intercept numbers of WC grain exceeded 200, the statistic data for the mean grain size of WC were reproduced. The discriminative minimal grain size of used LI method was 12 nm; the maximum intercept of WC grain was 109 nm; the average intercept of WC grains was 45 nm and the corresponding 3D mean grain size of WC was 70 nm which is agreeable with the XRD outcome.