Effects of annealing on the dielectric properties and microstructures of thin tantalum oxide film(25nm) deposited on p-type Si substrate with rf reactive magnetron sputtering were investigated. The leakage current density was remarkably reduced from $10^-8$ to $10^-12$ A/$mum^2$at the electric field of 2MV/cm after rapid thermal annealing(RTA) in $O_2$at $1000^{circ}C$, while little leakage reduction was observed after furnace annealing in $O_2$ at $500^{circ}C$. The structural changes of thin tantalum oxide film after annealing were examined using high resolution electron microscope(HREM). The results of HREM show that substantial reduction in the leakage current density after the RTA in $O_2$ can be attributed to crystallization and reoxidation of the thin amorphous tantalum oxide film.