Abstract: Single-wall and cup-stacked carbon nanotubes (CS-CNTs) were synthesized by a constrained thermal plasma jet at atmospheric pressure using methane as carbon source and Fe₂O₃ as catalyst. The plasma jet flowed through a graphite cylinder directly into a graphite bowl where its flow was interrupted and carbon was deposited. Carbon was also deposited on a ceramic filter tube behind the bowl. Deposits at the two locations were characterized by scanning and transmission electron microscopy and differential thermal analysis. The carbon on the filter tube consisted of defect-free single-walled CNTs (SWCNTs), while that in the graphite bowl consisted of high-purity CS-CNTs. Gas flow has a significant influence in the structure of the products. Highly crystalline SWCNTs were formed under laminar gas flow and the CS-CNTs were produced under turbulent gas flow. Diameter and purity of the CS-CNTs increases gradually toward the inner cavity of the graphite bowl due to a temperature increase.