Two human Fab antibodies against avian influenza A (H5N1) virus were obtained by panning a H5N1 patient-derived antibody phage library using purified virions of the H5N1 patient isolate A/Anhui/1/2005 and HA protein of the H5N1 reference viruse A/Viet Nam/1203/2004. After testing the binding properties and antiviral function to H5N1 virus, the selected Fab antibodies were converted to full human IgG antibodies with recombinant baculovirus/insect cell system. Both mAbs, AVFluIgG01 and AVFluIgG03, bound to HA in immunofluorescence assay (IFA) without cross-reaction with the other substypes of influenza A viruses (H1N1, H3N2). The cross-reactivity of the two antibodies for different strains of H5N1 was tested in vitro by micro-neutralization assays. In vitro, mAb AVFluIgG01 potently neutralized not only the selected well-characterized Clade 2 H5N1 viruses isolated from mainland of China except A/Guangdong/1/2006, but also the Clade 1 representative isolate A/Viet Nam/1203/2004; and AVFluIgG03 neutralized all the selected Clade 2 H5N1 viruses isolated from mainland of China, but had no neutralizing activity with the Clade 1 H5N1 virus A/Viet Nam/1203/2004. The results bring new prospect for the prophylaxis or treatment of H5N1 virus infection and may provide a clue for novel vaccine development.