Electrical properties and leakage current mechanisms of amorphous and polycrystalline tantalum pentoxide (Ta2O5) films were studied. Ta2O5 thin films were deposited on Pt/SiO2/n-Si substrate by reactive magnetron sputtering and then annealed at temperatures ranging from 500 to 800 °C for 30 min in O2. The as-deposited films were amorphous and showed good electrical properties in terms of a dielectric constant of about 31 and leakage current density of 10-10 A/cm2 at 100 kV/cm. The leakage current flowing through Ta2O5 film increases from 10-10 to 10-7 A/cm2 following the annealing. The conduction mechanism at low electric fields (100 kV/cm) is due to Ohmic conduction. On the other hand Schottky mechanism dominates at the intermediate fields (100-350 kV/cm) and Poole-Frenkel becomes predominant at high fields (>350 kV/cm). The increase in leakage current density in the crystallized film is due to Si penetrated into the Ta2O5 grain and grain boundary from the underlying SiO2/n-Si substrate.