The accessory protein Pol gammaB of the human mitochondrial DNA polymerase stimulates the synthetic activity of the catalytic subunit. Pol gammaB functions by both accelerating the polymerization rate and enhancing polymerase-DNA interaction, thereby distinguishing itself from the accessory subunits of other DNA polymerases. The molecular basis for the unique functions of human Pol gammaB lies in its dimeric structure, where the Pol gammaB monomer proximal to Pol gammaA in the holoenzyme strengthens the interaction with DNA, and the distal Pol gammaB monomer accelerates the reaction rate. We further show that human Pol gammaB exhibits a catalytic subunit- and substrate DNA-dependent dimerization. By duplicating the monomeric Pol gammaB of lower eukaryotes, the dimeric mammalian proteins confer additional processivity to the holoenzyme polymerase.