The core protein of classical swine fever virus (CSFV) is a nucleocapsid protein localized in the inner part of virion. The nucleolus localization of nucleocapsid protein of virus in family Flaviviridae plays an important role in virion packaging and virus replication. In this study, recombinant eukaryotic plasmids expressing CSFV Core protein, its truncated and amino acids mutants fused with EGFP were constructed, respectively. Subcellular localization of core protein and its mutants in the transfected PK15 cells were observed under confocal microscopy. We successfully identified the nucleolus localization sequence PESRKKL and its critical amino acids R76K77 in Core protein. Our work will further our understanding of the structure and function of CSFV Core protein and its roles in virion packaging and virus replication.