Carbon nanotubes (CNTs) were synthesized at 650-770°C using Co/MgO and Co-Mo/MgO as catalysts and ethylene as carbon source. The dependence of the carbon yield on Mo amount in catalyst, synthesis temperature and time were investigated, and the structures of CNTs were characterized using FESEM, TEM, and Raman spectroscopy. It was found that CNTs with "bamboo-like" structure were synthesized using Co-Mo/MgO catalysts, in contrast to typical multi-walled carbon nanotubes with hollow inner tube obtained from Co/MgO catalyst. Addition of 5-10% Mo in catalysts resulted in the highest carbon yield, e.g. 5500% for 60 min synthesis, which is about 20 times higher than that of Co/MgO. The addition of Mo also extended the lifespan of catalyst for the growth of CNTs. TEM observation showed that the growth of CNTs was in a root-growth mode. The structure of CNTs obtained from Co-Mo/MgO catalysts was explained based on the growth mechanism of CNTs.