To investigate the cause of death of a pregnant woman with undefined pneumonia reported from the People's Hospital of Tongling City in Anhui Province on November 8, 2005, the patient's tracheal aspirates and serum samples were collected and tested by RT-PCR and Real-time PCR to detect viral nucleic acids of HA of A/H5N1, A/H7N7, A/H9N1 and A/M. Tracheal aspirates were inoculated into special pathogen free (SPF) embryonated eggs for cultivation and identification of virus. The HA gene of the virus was sequenced and analyzed. Serum samples were tested by HI assay to detect antibody of H5N1. The results showed that HA gene of A/H5N1 virus and A/M were positive in tracheal aspirates by both PCR tests. The serum sample collected on Nov. 9 was A/M gene positive by Real-time PCR. The analysis of HA gene of A/AnHui/1/2005 sequence showed that the receptor specificity and the connecting peptide between HA1 and HA2 were still avian influenza origin. The HI antibody of H5N1 was negative at 7th, 8th, 9th d of disease onset. This undefined pneumonia case was confirmed as the first pregnant woman case of avian influenza (H5N1) virus infection by etiology in the mainland of China.