Abstract: By using the quantum-chemical CNDO/2 method, the mechanism of inhibition of ribonucleotide reductase by aryl hydroxamic acids has been studied. It is ascertained that the mechanism of inhibition is metal chelation. Furthermore, a new metal chelation mechanism for aryl hydroxamic acids is suggested that not only could the--CONHOH moiety chelate metal ion in ribonucleotide reductase to form uniposition chelation, but also the two adjacent hydroxyl or amino groups on benzene ring could chelate metal ion to form biposition chelation. This mechanism reasonably accounts for some experimental facts which can not be explained by the traditional metal chelation mechanism.