ORF11 (ac11) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a highly conserved baculovirus gene whose homologs are found in all lepidoteran Group I NPV, but its function is unknown so far. To determine the role of ac11 in baculovirus life cycle, ac11 knock-out mutant, Ac11KO, was constructed using the plasmid capture system (PCS). Real-Time PCR analysis showed that ac11 transcript was first detected at 6 h post-infection (p.i.) and accumulated to maximum at 48 h p.i., indicating that ac11 is belong to late gene. When the genomic DNA of Ac11KO was transfected into Sf9 cells, viral replication was restricted to a cell transfected originally. While viral transmission of the Ac11KO was not observed in Sf9 cells, production of budded virus (BV) in Sf9 cells transfected with Ac11KO was observed by transmission electron microscopy (TEM). These results suggest that the ac11 is essential for AcMNPV to produce infective BV.