Peptidylarginine deiminase IV (PAD4) catalyzes the conversion of an Arg residue to a citrulline residue in various proteins. In particular, citrullination of histone subunits, such as H2A and H3, by PAD4 is thought to be related to rheumatoid arthritis. However, the details of the citrullination mechanism of histone H2A and H3 are not yet well known. Moreover, the effects of N-terminal acetylation on histone subunits with respect to PAD4 recognition have not yet been studied. To further study the mechanism of PAD4 recognition of histone H2A and H3 subunits, a series of the N-terminal peptides was chemically synthesized and the citrullination sites were identified using MALDI-TOF/MS. N-terminal acetylation of histone H2A was not significant with respect to PAD4 recognition in vitro, but the acetylation of H3 peptide had a significant effect on PAD4 recognition in vitro, resulting in predominant citrullination at the Arg2 residue.