Poly-Si films are grown by ultrahigh vacuum chemical vapor deposition (UHV-CVD) with plasma-seeding on SiO2. The preferred grain orientations are investigated as a function of thickness, source gas flow rate, and growth temperature. The poly-Si films show (100) preferred orientation in a wide range of growth conditions. The (100) orientation increases as thickness and flow rate increases and decreases as temperature increases. UHV-CVD and other deposition methods are compared in order to estimate the role of hydrogen and oxygen as factors controlling the preferred grain orientation.