Ultrafine single-walled carbon nanotubes (SWNTs) are successfully fabricated at cryogenic temperature by a UV irradiation assisted template-growth method.In situ Raman spectroscopy is used to monitor the growth process in real time.The growth process is studied at different temperatures and efficient growth is observed even at 80K.The growth rate increases rapidly with temperature above 273K while is not affected by temperature under 273K.It is also found that methane can serve as the carbon source.Evidence supported that the growth is a photoinitiated radical chain polymerization.