Objective To isolate and identify the pathogen causing the inclusion body hepatitis (IBH) and hydropericardium syndrome (HPS) in chicken in Guangdong Province, and study its molecular characteristics.Method A virus was isolated from specific pathogen-free(SPF) chicken embryo inoculated with liver suspension from affected chickens. The virus was purified by limiting dilution method and used for pathogenicity test. Hexon, fiber-2, ORF29 and ORF19 genes were amplified by PCR, and analyzed for molecular characteristics.Result Sequence analysis of hexon gene revealed the strain, named as WM-XC, belonged to group I fowl adenovirus serotype 4 (FAdV-4). This isolated strain was pathogenic based on the evidence that the mortality rate of SPF chicken reached 90% after virus inoculation, the hydropericardium-hepatitis syndrome was observed, and acidophilous inclusion body was found in the pathological section of liver. The fiber-2 gene sequence of WM-XC had 96.1% and 97.0% similarities with the corresponding sequences of ON1 and KR-5, and had 100% similarity with fiber-2 genes of fowl adenovirus isolates collected in China since 2015. There was a 33 bp deletion in ORF29 gene of WM-XC compared with JSJ13 and JN13 strains isolated in 2013 in China. Similar to isolates collected in China since 2015, WM-XC lacked ORF19 compared with classical FAdV-4 strain ON1 and KR-5.Conclusion The isolated group I FAdV-4 strain and other isolates collected in China in recent years were distinct from classical FAdV-4 strains in their fiber-2, ORF19 and ORF29 gene sequences.