The effect of hydrogen dilution on microstructure of in situ polycrystalline Si (poly-Si) films grown by catalyzer enhanced chemical vapor deposition (CECVD) has been investigated. It was shown that microstructure of the Si films grown at low substrate temperature (<300 °C) in CECVD was strongly affected by hydrogen dilution ratio (H@sub 2@/SiH@sub 4@). In addition, a secondary ion mass spectroscopy (SIMS) depth profile of the in situ poly-Si film grown by CECVD at SiH@sub 4@/H@sub 2@ (1/40 sccm) exhibited much lower hydrogen concentration than dehydrogenated amorphous Si film grown by conventional plasma enhanced chemical deposition (PECVD). These results indicated that the CECVD technique is a promising candidate to grow high quality in situ poly-Si films on glass or flexible substrate for low-temperature poly-Si (LTPS) and flexible displays.