High-quality single-walled carbon nanotubes (SWNTs) have been synthesized over Fe-Mo/MgO catalyst by catalytic decomposition of ethylene at 800degreesC. The produced carbon material primarily consists of a SWNT bundle with few defects and a very small amount of amorphous carbon coating. The diameter of an individual SWNT is in the range of 0.7-2.8 nm. The as-synthesized SWNTs have a high yield of over 550% relative to the weight of Fe-Mo metal in the Fe-Mo/MgO catalyst. Our results show that ethylene can be a very ideal carbon source for the synthesis of SWNTs. We suggest that catalytic decomposition of ethylene over Fe-Mo/MgO catalyst can promise a large-scale production of high-quality SWNTs.