The microcrystalline silicon films at different growth stages were deposited by plasma-enhanced chemical vapor deposition (PECVD). The reflectivity of grazing incidence X-ray from synchrotron radiation has been applied to investigate the evolution of surface roughness of these thin films. By study of surface morphology of microcrystalline silicon (μc-Si:H), we understand their growth kinetics and growth mechanism. The results show that the growth exponent β is 0.21±0.01 and 0.24±0.01 for μc-Si:H films deposited on glass substrate at fixed substrate temperature, Under the following condition of electrode distance, pressure, rf power density, H2 dilutied at 200 ℃ to be 2 cm, 6.66×102　Pa and 0.22 W/cm2, 99% and 98%, respectively. According to the KPZ model in the PECVD case the growth mechanism of the μc-Si:H films is a finite diffusion growth.