Human cytomegalovirus encodes an unusual protein kinase UL97 which can phosphorylate exogenous substrates, including histone H2B and nucleoside analogs such as ganciclovir. The previous result interestingly showed that the peptides phosphorylated by UL97 have K/R at the 5 positions (P+5) downstream from the pSer. To confirm the importance of the basic residue in the position, we used two peptide libraries, 4S4K (MAXXXXSXXXXKXANNN) and 4S6N (MAXXXXSXXXXXXNNN). The activity of phosphorylation by UL97 was higher in the peptide library 4S4K than 4S6N, suggesting the importance of basic residue at P+5 position. The screening with a peptide library 4S4K showed slight tendencies for N in the P+1 and P+2, M in the P+2, K in the P+4 and P+6 positions and several amino acids in the other positions. This result will give information to develop an optimal peptide for screening a novel UL97 inhibitor.