Polycrystalline silicon films were prepared from SiCl4 diluted with hydrogen by plasma-enhanced chemical vapor deposition at a low temperature of 250℃. The effect of hydrogen dilution on their structure and optical properties were investigated. It was found that the effect of hydrogen on the growth behavior of the films deposited using SiCl4+H2 are completely different from that of using SiH4/H2. The crystalline fraction increases with decreasing the hydrogen dilution ratio R and reaches a maximum value of 85% at a low hydrogen dilution. However, further decreasing hydrogen dilution ratio leads the crystallinity to deteriorate and a phase transition between microcrystalline silicon and amorphous silicon is observed. The optical band gap gradually decreases from ～1.5eV to ～1.2eV and then increases up to 1.8eV with the decrease of the hydrogen dilution ratio. The deposition rate initially increases and then decreases with decreasing hydrogen dilution ratio, and no film is formed by using pure SiCl4. According to these results, we conclude that under the condition of optimum hydrogen dilution ratio, the enhanced crystalline fraction and the increase in grain size are attributed to the Cl radicals which play important roles in the low-temperature growth of crystalline silicon films.