The formation of microcrystalline silicon films has been studied under both low and high pressures with plasma enhanced chemical vapor deposition (PECVD). Under low pressure, the growth rate of films may be enhanced by increasing the silane fraction in the reaction gas, or by increasing the input power density. However, both of these methods tend to inhibit the growth of microcrystalline films and promote the growth of amorphous phase. Under high pressure, the microcrystalline can be formed as the silane is depleted by increasing the power density. In this paper, the mechanism of microcrystalline formation of the films has been analyzed based on the observations of the optical emission intensities of H-alpha and Si* in the deposition plasma. It has also been found that the crystallinity near the interface between the deposited film and the substrate can be significantly improved for films deposited under high pressure.