Although vaccination has been hugely successful in protecting birds against infection by the New castle disease virus (NDV), newly-emerged highly virulent strains have been found to overcome established immune protection and threaten the poultry industry. The need to improve the immunization efficacy is, therefore, urgent. Here, we tested the potential immunostimulatory adjuvant activity of the adenoviral-expressed recombinant chicken granulocyte monocyte colony stimulating factor (rchGM-CSF) in an inactivated Newcastle Disease Virus (NDV) vaccine. 126 commercial layer chicks, divided into six groups, were first vaccinated at day 7, followed by a subsequent boost and later an intramuscular challenge at day 21 and 35 respectively. rchGM-CSF expressed by adenovirus raised NDV-specific hemagglutinin-inhibition (HI) titers from 10 to 12 (log2) and significantly upregulated the production of interferon α/β/γ (IFN-α/β/γ), interleukin-4 (IL-4) and major histocompatibility complex II (MHC-II) in spleens. Crucially, chicks inoculated with the inactivated NDV vaccine plus the rchGM-CSF adjuvant displayed only mild clinical signs, lower tissue viral loads, fewer tissue lesions, and decreased mortality and viral shedding than those in the group immunized with the vaccine alone. Our present work has demonstrated that chicken GM-CSF may act as an enhancer in the orchestration of host immune responses induced by the inactivated NDV vaccine. The molecule, expressed by an adenovirus, has the potential to be used as an immune adjuvant to improve protection by NDV vaccination.