Filamentous carbons were prepared by the catalytic pyrolysis of CH4 over the nickel supported catalysts, and their structure and reactivity were studied by using SEM, TEM, Raman spectroscopy, TPH, TPO and TPCO2. The catalysts were prepared by impregnation of SiO2 with Ni(NO3)(2).6H(2)O or Ni(acac)(2) and are denoted as Ni/SiO2-N and NUSiO2-A, respectively. There was observed an optimum temperature around 600 degreesC for the formation of filamentous carbons on both catalysts; the amount and the diameter of filamentous carbons formed on the catalyst depended on the preparation method of the catalysts. TEM observation showed that the size of Ni metal particles was < 10 nm on Ni/SiO2-A and between 10 and 50 nm on Ni/SiO2-N. The carbon/Ni atomic ratio after the pyrolysis was much larger on Ni/SiO2-N than on Ni/SiO2-A, and the diameter of filamentous carbons is smaller by ca. 10 nm on Ni/SiO2-A than on Ni/SiO2-N. Raman spectroscopy also showed the smaller size of carbon on Ni/SiO2-A than on Ni/SiO2-N. According to the results obtained by TPO, TPH and TPCO2 of the carbon species after the pyrolysis of CH4 at 600degreesC, the carbon deposited on Ni/SiO2-A is more reactive to oxygen, hydrogen and carbon dioxide than that deposited on Ni/SiO2-N.