We report the variation of yield and quality of carbon nanotubes (CNTs) grown by chemical vapor deposition (CVD) of methane on iron oxide-MgO at 900-1000 degrees C for 1-60 min. The catalyst was prepared by impregnation of MgO powder with iron nitrate, dried, and calcined at 300 degrees C. As calcined and unreduced catalyst in quartz reactor was brought to the synthesis temperature in helium flow in a few minutes, and then the flow was switched to methane. The iron oxide was reduced to iron nanoparticles in methane, while the CNTs were growing. TEM micrographs, in accordance with Raman RBM peaks, indicate the formation of mostly single wall carbon nanotubes of about 1.0 nm size. High quality CNTs with I-G/I-D Raman peak ratio of 14.5 are formed in the first minute of CNTs synthesis with the highest rate. Both the rate and quality of CNTs degrades with increasing CNTs synthesis time. Also CNTs quality sharply declines with temperature in the range of 900-1000 degrees C, while the CNTs yield passes through a maximum at 950 degrees C. About the same CNTs lengths are formed for the whole range of the synthesis times. A model of continuous emergence of iron nanoparticle seeds for CNTs synthesis may explain the data. The data can also provide information for continuous production of CNTs in a fluidized bed reactor. (C) 2011 Elsevier B. V. All rights reserved.