Although the structure-function of hepatitis B virus (HBV) core protein has been investigated by numerous HBV core mutants, functions of many regions in the core protein are still remained to be identified. In this report, it was found that point mutations in the 113th and 117th negative-charged amino acids in the 5th helix region of the HBV core strongly affect pregenomic RNA (pgRNA) encapsidation. These mutations were introduced by site-directed mutagenesis. The following results were obtained from analyses of the mutants. First, endogenous polymerase activity (EPA) was assayed and activity was not detected only in the two mutants, E113K and E117K. Second, the pgRNA encapsidation level of each mutant related to a change in charge of two amino acid sites was evaluated. Mutations in the 113th and 117th amino acids into uncharged amino acids reduced pgRNA encapsidation levels. Moreover, changes of the two amino acids into positive-charged amino acids almost completely reduced pgRNA encapsidation levels. To test whether the mutant core proteins assembled into normal capsid particles, the assembly of the mutant core proteins was seen. However, none of the changes in the 113th and 117th amino acids affected capsid formation. From this data, it can be inferred that the above two amino acids in the 5th alpha-helix in the HBV core protein are important for pgRNA encapsidation.