Studying the sources of uncertainty can improve the accuracy of the measurement result. There are mainly two ways of estimation measurement on uncertainty depending on the purpose of the estimation and the available data. This study evaluated the uncertainty of chromium content in fertilizers using both the TOP-DOWN approach and the BOTTOM-UP approach. The measurement uncertainty of chromium determined by the TOP-DOWN approach was 17 mg/kg，while that determined by the BOTTOM-UP approach was 21 mg/kg. The results indicated that the measurement uncertainty evaluated by the two approaches were essentially consistent. With the accumulation of a large amount of check sample data，the TOP-DOWN approach proved to be simpler，required less calculation，was easier to operate，and could better reflect the precision and accuracy levels of long-term laboratory testing.