Pyramid shaped pyrolysis flame is a new method to synthesis carbon nanotubes (CNTs). Oxy-acetylene flame was used as the source of heat, CO as the source of carbon, and iron pentacarbonyl (Fe(CO)5) as the source of catalyst. Field emission scanning electron microscope (FE-SEM) and High resolution transmission electron microscopy (HR-TEM) were used to illustrate the experimental results. Our results show straight, uniform and high degree graphitization CNTs were produced whereas lots of deformity tubes were present, as bamboo-shaped tubes, filled tubes, diameter changed tubes, irregular tubes, etc. In order to understand the synthesis mechanism of deformity tube, many models were presented for the different shape tubes. Synthesizing bamboo-shaped tubes maybe decide by capillary siphon and the low melting state of catalyst particles. Besides capillary siphon and the low melting state of catalyst particles, filled tubes mainly connect with the continuous supply of catalyst atom clusters. The diameter changed tubes maybe influenced by the diameter changing of catalyst particles and the action of other materials. Irregular tubes maybe affect by temperature and other factors.