Cation-substituted PZT thin films are more desirable for ferroelectric memory applications due to their lower coercive field, higher resisitivity and lower dielectric memory-aging rate compared to PZT thin films. The resulting effect of cation-substitution can be explained on the basis of defect chemistry in a perovskite lattice. In this study, La and Nb were chosen as substituents and their effects on fatigue behavior and leakage properties of PZT-base thin films were investigated. La- or Nb-substituted PZT thin films were deposited by the sol-gel processing method on Pt electrode. The Zr/Ti ratio was fixed as 40/60 with the tetragonal perovskite phase. The excess Pb added onto the starting precursor was fixed to 15 wt.%. Each sol-gel process condition and heating process were optimized based upon its thermal analysis result. The surface microstructure, crystallinity, ferroelectric properties, and leakage characteristics were investigated. It is shown through CV and I-V characteristics that both A-site (La) and B- site (Nb) substituent play a role in development of electrical properties. However. cation substitution induces lattice site defect to maintain a charge neutrality. In case of Nb, Pb-deficient pyrochlore phase was formed due to A-site vacancies (Pb vacancies). PNZT films showed the degradation of ferroelectric properties.