Abstract: Human serum albumin (HSA) is the most abundant carrier protein in human blood plasma and can bind various endogenous metabolites, metal ions and medicines. Studying on the interactions between HSA and anticancer drugs, we can get the useful information about how the drugs transported and metabolized in body, which is very important for drug design and development. Here, we studied on the interactions between human serum albumin and (h⁶-cymene)RuCl(en)PF₆ (1), (h6-biphenyl)RuCl(en)PF₆ (2) by LC-MS, and found that complex 1 bonded to thiol group of Cys34 of HSA which was subsequently oxidized to sulfinic acid, while complex 2 did not bind to Cys34 due to the steric hindrance from the biphenyl ligand. However, both complexes 1 and 2 can covalently bind to histidine and methionine residues on the surface of the protein