Abstract: The present invention relates to mutant DNA polymerases and their genes isolated from Thermococcus sp. More specifically, the present invention relates to mutant DNA polymerases which are originally isolated from Thermococcus sp NA1. strain and produced by site-specific mutagenesis, their amino acid sequences, genes encoding said mutant DNA polymerases, their nucleic acids sequences, recombinant vectors containing said nucleic acids sequences, host cells transformed with thereof and methods for producing mutant DNA polymerase protein by using thereof. As mutant DNA polymerases according to the present invention have increased processivity by site-specific mutagenesis on exonuclease active site compared to wild type DNA polymerase, the present invention is broadly applicable for PCR in various molecular genetic technologies.