Carbon Nanotubes (CNTs) are excellent material with a variety of physical, electrical, and mechanical properties. The average aspect ratio is evaluated for samples synthesized at different temperature and time of precursor flow. The experiments were conducted on calcium carbonate (CaCO3) substrate with iron-cobalt (Fe-Co) as a catalyst. Acetylene (C2H2) was used as a precursor gas and argon (Ar) as a carrier gas. The experiments were performed at temperatures ranging from 600 0C to 900 0C in step of 100 0C with time of precursor flow varying from 15 minutes to 75 minutes in the step of 15 minutes. The partial pressure of C2H2 and Ar kept constant during synthesis of CNT. The mass flow rate of C2H2 and Ar were 2 liters/hour and 60 liters/hour respectively during synthesis of CNT. The characterization of CNT was done with SEM, EDX, and TEM microscopes. Average aspect ratio of CNT is estimated using image analysis software. Most of nanotubes grown present an average diameter of 40 nm. The minimum and maximum diameter found are12.5 nm and 106 nm respectively. It is observed that average aspect ratio ranges from 25 to 58.