Objective To study the pathogenic prevalence and genotypes of astrovirus among children under 5 years old hospitalized with diarrhea in Tianjin. Methods A total 837 stool specimens were collected from children with diarrhea hospitalized in Tianjin children's hospital from May 2008 to April 2009. Astrovirus antigens were detected using ELISA and the postive specimens were inoculated in CaCo-2cells. After the CPE caused by virus were observed, the total RNA of virus was extracted, then the genomc fragments of the strains were amplified by using RT-PCR and confirmed by sequencing of the RT-PCR products. Detection of rotavirus was employed by Colloidal Gold Device. Results Astrovirus antigen was found positive in 3.0% of the patients. The coinfection rate of astrovirus and rotavirus was 0. 7% (6/837).Ninety-six persent of children with astrovirus diarrhea were younger than 2 years of age, Forty-eight persent of children with astrovirus diarrhea were younger than 6 months. The astrovirus infections occurred mainly between August 2008 and April 2009. Of the 21 astrovirus positive specimens, 11 cases were successfully identified by RT-PCR and they were all serotype 1. Conclusion Astrovirus is a major cause of nonbacterical diarrhea between 2008 and 2009 in Tianjin, and the predominant serotype is type 1.