This study examined carbon nanotubes (CNTs) with various outer diameters produced by the catalytic decomposition of acetylene over Co-loaded zeolite Y. The CNTs were grown at differed reaction temperatures, reaction times, and acetylene concentrations. In addition, the effect of the amount of Co dispersed over zeolite Y used as a support was determined. The shape and diameter of the synthesized CNT were identified by SEM and TEM analyzers. As a result, CNTs with various outer diameters were synthesized successfully. The average outer diameter of the synthesized CNTs increased with increasing amount of Co dispersed over zeolite Y regardless of the reaction temperature and reaction time. The outer diameter did not change with acetylene concentration, and the acetylene concentration was fixed to 10 cm3/min. Most of the CNT had large surface areas, >400m2/g. The surface area increased with increasing outer diameter of the CNT until the outer diameter reached 60 nm but decreased with further increases in outer diameter.