Ferroelectric thin films of YMnO3 and Y2O3 insulator were deposited on Si(100) substrates by metal-organic chemical vapor deposition. When the YMnO3 thin films were annealed in ambient oxygen, the secondary phases of orthorhombic YMnO3 (o-YMO), Y2O3 and Y2Mn2O7 were observed, along with the primary phase of hexagonal YMnO3 (h-YMO). On the other hand, the films annealed in vacuum (100 mtorr) crystallized to an h-YMO single phase, without an apparent secondary phase. When the gate voltage swept from +5 to -5 V, the capacitor had a hysteresis curve with a clockwise rotation, which indicates ferroelectric polarization switching behavior. The memory window of the Pt/YMnO3/Y2O3/Si gate capacitor annealed in vacuum at 850 degreesC is 1.8 V. The typical leakage current density of the films annealed in ambient oxygen and vacuum are approximately 10(-3) and 10(-7) A/cm(2) at an applied voltage of 5 V, respectively. The annealing atmosphere plays a critical important in the crystallinity and electrical properties of YMnO3 thin films.