YMnO3 thin films deposited on Si (100) substrate by rf-sputtering were annealed with various conditions. YMnO3 films annealed in a furnace had a c-axis preferred orientation and the films annealed in a rapid thermal processor (RTP) had random orientations. However, cracks were observed in the highly c-axis oriented YMnO3 films. YMnO3 films annealed in the furnace showed poor ferroelectric characteristics. However, YMnO3 films annealed in the RTP showed a ferroelectric C-V hysteresis with 1.5 V memory window at 0.2 V/s sweep rate. Since the thermal expansion of a-axis is five times higher than that of c-axis in the YMnO3 thin films, the c-axis oriented thin films are expected to be easily cracked during the post-annealing process. Moreover, the rapid thermal annealing process effectively suppressed the increase of a native SiO2 thickness in the YMnO3/Si structure.