Abstract: By using a reciprocating sliding wear test machine, friction and wear behaviors of human tooth enamel opposite to titanium alloy have been investigated in citric acid solution (pH=3.2). The results showed that human tooth enamel exhibited different tribological behaviors in two different media, i.e. citric acid solution and artificial saliva. Due to the enamel surface softening and demineralization caused by acid-attack, the friction coefficient was lower and fluctuated more significantly in citric acid solution than in artificial saliva. The worn surface was characterized mainly by adhesive delamination. Wear volume almost increased linearly with the number of cycles in citric acid solution, which was bigger than that in artificial saliva.