Human serum albumin (HSA) is the most abundant plasma protein in the human body with a plasma concentration of 0.6mM. HSA plays an important role in drug transport and metabolism. Enzymatic activity of HSA on different substrates or drugs has been studied and documented. The structural mechanism of this activity, however, is unknown. In this study, we have determined the crystal structures of HSA-myristate in a complex of aspirin and of salicylic acid, respectively. The crystal structure of HSA-myristate-aspirin illustrates that aspirin transfers acetyl group to Lys199 and is hydrolyzed into salicylic acid by HSA. The hydrolysis product, salicylic acid, remains bound to HSA at a similar location, but it shows a very different orientation when compared with the salicylic acid in the HSA-myristate-salicylic acid ternary complex. These results not only provide the structural evidence of esterase activity of HSA, and demonstrate the conformational plasticity of HSA on drug binding, but also may provide structural information for the modulation of HSA-drug interaction by computational approach based on HSA-drug structure.