The effects of H2O on the synthesis singlewall carbon nanotubes(SWCNTs)by decomposing CH4 in Ar at 800℃ were studied by TEM and Raman scattering. It is shown that the water existing in the reacting gas increases the percentage composition of the SWCNTs in the products. Preliminary analyses indicate that the water is involved in the purification course, and the generation of the amorphous carbon is brought about. Furthermore, the water can affect the distribution of the SWCNTs'diameter.