Abstract: The inclusion size and dispersion were regarded as the value of the cleanliness of continuous casting slab, which can be measured by metallographic method. The uncertainty that influencing the cleanliness measuring results was analyzed from the aspects of repeatability of experiment, metrology performance of metallographic microscope, manual counting of inclusion, metrology performance of digital caliper and so on, and the uncertainty components were calculated and synthesized. Finally, the uncertainty report for the cleanliness measuring results was obtained. The results show that the relative expanded uncertainty of inclusion size of continuous casting slab was 9. 3%, the relative expanded uncertainty of inclusion dispersion distribution was 7. 6%, and the inclusion factor k=2.