We report the significant improvement of the ferroelectric properties of BiFeO3 thin film through control of electrical leakage by Nb doping. A very large remanent electrical polarization value of 80 mu C/cm(2) was observed in Bi0.8La0.2Nb0.01Fe0.99O3 thin film on Pt/Ti/SiO2/Si substrate. The doping effect of Nb in reducing the movable charge density due to oxygen vacancies in BiFeO3 was confirmed by the dielectric measurements. A very small loss was observed in the Nb and La codoped BiFeO3 thin film. As well as the improvement in the ferroelectric properties, the magnetic moment was also enhanced due to the doping of La.