Pyramid sharped pyrolysis flame is a new method for carbon nanotubes synthesis. Oxy-acetylene flame outside the frustum of pyramid sharped reactor provides the necessary high temperature environment, while CO is used as the source of carbon, iron pentacarbonyl (Fe(CO)5) as the source of catalyst precursor in reactor. Inside, the mixture of CO, H2, and Fe(CO)5 will burn incompletely after initial reaction under high temperature. Immerging a sampling substrate into the incomplete burning flame can gain carbon nanotubes. Field emission scanning electron microscope and High resolution transmission electron microscopy were used to illustrate the results of experimental. By adjusting the distance between the oxy-acetylene flame jet and the synthesis zone, the purpose that only changing one factor of synthesis or pyrolysis temperature while the other one constant was achieved, then respectively studied the effects of them on experimental. The best synthesis temperature in experimental is about 590°C and pyrolysis temperature is about 1000°C. Effects of sampling time were also studied. 300s is economic for carbon nanotubes synthesis. Synthesis process of carbon nanotubes was discussed starting from the sampling time. The first 60s is the time for the temperature rise of sampling substrate. After the decalescence and thermal dissipation of substrate is balance, the temperature of substrate is fixed, and the main synthesis phase of carbon nanotubes begin.