[Objective] Fowl adenovirus (FAdV) infection in chicken flocks is very common in China currently. Because FAdV transmits both horizontally and vertically in chickens, it is essential and urgent to determine the contamination of attenuated vaccines produced in chicken embryo eggs by FAdV and its risk to induce epidemic of Hepatitis-Hydropericardium syndrome (HHS) caused by FAdV. The objectives were to ascertain the contamination of an attenuated live Newcastle disease virus (NDV) vaccine produced by Chinese veterinary drug company by FAdV and to investigate the possible reasons for current prevalence of HHS caused by FAdV in chicken flocks in China. [Methods] FAdV-specific PCR was employed to detect FAdV from the attenuated live vaccine of NDV La Sota strain. PCR positive samples were inoculated into chicken embryo eggs for the isolation of FAdV. Sequencing, identity analysis and the phylogenetic tree construction were performed to analyze the molecular characteristics, genotyping and serotyping of the isolates. [Results] A FAdV strain was obtained and named L160962 that shared the highest identity with serotype 4 FAdVs in the 52k and hexon genes according to homology analysis. Among the strains investigated, isolate L160962 shared identities at a level of 99.9%?100% with Chinese strains HN/151025 and CH/HNJZ/2015, 99.3% with Austrian strain KR5, and only 79.0%?83.3% with members in species A, B, D and E in the sequences of 52k and hexon. Phylogenetic analysis showed that isolate L160962 was clustered into serotype 4 within the species C with an identity higher than 99.9% and most close to the Chinese strains, suggesting that isolate L160962 is similar to the prevalent strains in China. [Conclusion] Our studies observed contamination of attenuated vaccines produced in chicken embryo eggs by serotype 4 of FAdV, which may be one of the reasons that lead to FAdV infection prevalent in chicken flocks in China.