Abstract: Multi-walled carbon nanotubes (MWCNTs) have been synthesized by chemical vapor deposition using xylenes as carbon precursors and ferrocene as a catalyst. The stability of the purified MWCNTs was investigated, their properties were characterized by SEM and volume electric resistivity. Results showed that MWCNTs exposed to air under ambient temperature will be slowly oxidized and shortened. The extent of oxidation increases with time, and MWCNTs were almost completely transformed into amorphous carbon after 15 d. The volume electric resistivity also varied as a result of oxidation. These results indicate that as-prepared MWCNTs should be kept in a well-closed container to prevent them from oxidization.