We performed in situ X-ray absorption near edge structure (XANES) measurements for Co and Ni catalysts during single-walled carbon nanotube (SWCNT) growth at 650℃. XANES results showed that both catalyst particles were reduced to metallic after heating at the growth temperature. After the ethanol gas irradiation started, Co catalysts became carbides, but Ni catalysts were kept metallic. These results were consistent with environmental transmission electron microscopy (TEM) observations and also a theoretical calculation.