A pressure-dependent transition (5kPa-100kPa) of carbon nanotube (CNT) morphology in plasma enhanced chemical vapor deposition (PECVD) is presented. High-purity, vertically-aligned single-walled CNTs (SWCNTs) were synthesized only when PECVD was used at atmospheric pressure, while multi-walled CNTs were preferentially synthesized when the total pressure was lower than 20kPa. In the reduced pressure range, nanostructured catalysts were easily coagulated at the initial stage of CNT nucleation even if an excess supply of reactive species and high-energy ion bombardment were absent. If catalyst coagulation was avoided at the moment of CNT uncleation, SWCNTs were grown in the root growth regime even at 5kPa; however, the top CNT layer was contaminated by amorphous carbon, produced as a result of excess supply of reactive species.