Amorphous and nanocrystalline Si films were prepared using radio-frequency (13.56 MHz) plasma enhanced chemical vapour deposition with different SiH4/H2 ratios. The film residual stress was measured using curvature method as a function of SiH4 gas flow ratio (SiH4/(SiH4 + H2)). Results from high resolution scanning electron microscopy, X-ray diffraction and Raman scattering studies revealed the appearance of nanocrystallites with SiH4 gas ratio less than 3%. With a gradual decrease of SiH4 gas ratio, the film intrinsic stress increased significantly until SiH4 gas ratio reached 2%. Below that critical value, the film intrinsic stress decreased sharply. Different mechanisms of stress formation and relaxation during film growth were discussed, including ion bombardment effect, hydrogen and hydrogen induced bond-reconstruction, nanocomposite effects (nanocrystal embedded in an amorphous Si matrix). Results indicated that nanocomposite effect is the dominant factor in this maximum stress condition.