Catalyst plays a crucial role in determining the characteristics of carbon nanotubes (CNTs) produced by using thermal catalytic chemical vapor deposition (CVD). It is essential to investigate how the catalyst preparation affects the characteristics of CNTs because certain application demands specific size for optimum performance. This study reports the effect of the types of catalyst and the duration of the catalyst pre-treatment (wet etching time, dry etching time and ball milling) on the diameter of CNTs. The synthesized CNTs samples were characterized by scanning and transmission electron microscopy and Raman spectroscopy. Wet etching (2M hydrofluoric acid) time was varied from 1 to 2.5 hrs and the diameter range was found to be in the range of 23 to 52 nm. The diameter range for CNTs produced for 3 hrs and 5 hrs of dry etching treatment (with ammonia gas) are 38 to 51 nm and 23 to 48 nm, respectively. The diameter size of CNTs produced using Ni (14 to 25 nm) was found to be smaller than Fe (38 to 51 nm). There is a significant decrease in the diameter of CNTs by prolonging the wet etching period. Shorter and curly shaped CNTs can also be obtained by using Ni as the catalyst.