Core assembly modulators of viral capsid proteins have been developed as an effective treatment of chronic hepatitis B virus (HBV) infection. In this study, we synthesized novel potent pyrimidine derivatives as core assembly modulators, and their antiviral effects were evaluated in in vitro and in vivo biological experiments. One of the synthesized derivatives, compound <b>23h</b> (R<sup>1</sup> = MeSO<sub>2</sub>, R<sup>2</sup> = 1-piperidin-4-amine, R<sup>3</sup> = 3-Cl-4-F-aniline) displayed potent inhibitory effects in the in vitro assays (52% inhibition in the protein-based assay at 100 nM and an IC<sub>50</sub> value of 181 nM in the serum HBV DNA quantification assay). Moreover, treatment with compound <b>23h</b> for 5 weeks significantly decreased serum levels of HBV DNA levels (3.35 log reduction) in a human liver-chimeric uPA/SCID mouse model, and these effects were significantly increased when <b>23h</b> was combined with tenofovir, a nucleotide analogue inhibitor of reverse transcriptase used for the treatment of HBV infection.