The sudden emergence of severe acute respiratory syndrome coronavirus (SARS-CoV) has caused global panic in 2003,and the risk of SARS-CoV outbreak still exists.However,no specific antiviral drug or vaccine is available;thus,the development of therapeutic antibodies against SARS-CoV is needed.In this study,a nanobody phage-displayed library was constructed from peripheral blood mononuclear cells of alpacas immunized with the recombinant receptor-binding domain(RBD) of SARS-CoV.Four positive clones were selected after four rounds of bio-panning and subjected to recombinant expression in E.coli.Further biological identification demonstrated that one of the nanobodies,S 14,showed high affinity to SARS-CoV RBD and potent neutralization activity at the picomole level against SARS-CoV pseudovirus.A competitive inhibition assay showed that S14 blocked the binding of SARS-CoV RBD to either soluble or cell-expressed angiotensin-converting enzyme 2 (ACE2).In summary,we developed a novel nanobody targeting SARS-CoV RBD,which might be useful for the development of therapeutics against SARS.