The synthetic behaviors of carbon nanotubes (CNTs) by Fe/MgO catalysts were investigated in 0~90 wt.% range of MgO mixture ratios by catalytic chemical vapor deposition (CCVD) process. The CNTs were synthesized with 40 minutes of synthetic time, and 923 K of synthetic temperature using 0.1 L/min of ethylene gas and 1.0 L/min of hydrogen gas as synthetic and carrier gas, respectively. As the increase of synthetic temperatures and times, the diameters of CNTs become thicker. The carbon yield showed in a parabolic curve as MgO content increased and the maximum carbon yield was obtained at 30 wt.% of MgO. There were no obvious changes in the diameters of CNTs respect to the change of MgO content. Fe/MgO CNTs showed good crystalinity by High Resolution Transmission Electron microscope (HR-TEM) analysis. The behaviors of Fe/MgO CNTs have a tendency of depending on synthetic time and temperature rather than MgO content.