We investigated ferroelectric properties of 20nm-thick epitaxial Pb(Zr,Ti)O3 (PZT) ultrathin films prepared on SrRuO3 (SRO)/SrTiO3 (STO) by metalorganic chemical vapor deposition (MOCVD). When SRO with terrace ledges was used as a bottom electrode, 20nm-thick PZT ultrathin films with ferroelectricity were successfully obtained. These PZT films exhibited saturated hysteresis loops with remanent polarizations (Pr) of 29–33 μC/cm2 and coercive electric fields (Ec) of 340–370 kV/cm. On the other hand, when PZT films were grown on SRO without terrace ledges, PZT films showed no saturated hysteresis loops because of large leakage current. The 15nm-thick PZT ultrathin film exhibiting unsaturated hysteresis loop was also obtained.