High quality superconducting HgBa2CaCu2Ox (Hg-1212) thin films have been fabricated on SrTiO3 (STO) substrates using YBa2Cu3Oy (YBCO) as a seed-buffer layer (SBL). X-Ray diffraction indicates that the Hg-1212 thin films have a c-axis epitaxial structure with (100)(Hg-1212)//(100)(STO). The phi -scans and rocking curves demonstrate that the SBL results in a better orientation of the Hg-1212 films. Scanning electron microscopy (SEM) also shows that the film surfaces are more regular. The Hg-1212 thin films on YBCO SBL have a superconducting transition temperature T-c=115-124 K and a critical current density J(c)> 1x10(6) A/cm(2) at 77 K in zero field.