Abstract: Deactivated causes of Ni/AC catalyst for vapor phase carbonylation of methanol were investigated. The carbonylation reaction was carried out in a fixed bed reactor at pressurized condition in the presence of methyl iodide as the promoter. Several characterization techniques (XRD, XPS and ICP) were used to study catalyst change before and after reaction. The characterization results showed that the number of Ni0 active center decreased with the formation of NiI2, and that carbonylation active center Ni migrated continuously and accumulated on the surface of the catalyst forming the active center of carbon deposition, resulting in catalyst deactivation. Species and structure of carbon deposition on the catalyst were also investigated. Results showed that graphite carbon deposited on Ni/AC catalyst after reaction. The loss of Ni on the surface of support induced by the formation of Ni(CO)4 was confirmed by ICP-AES.