The effect of H-2 gas during the carbon nanotubes (CNTs) synthesis with CO-H-2 gas mixture was investigated using by mass measurement, and scanning electron microscopy (SEM). The maximum weight and yield of the synthesized carbon (C) were obtained when the mixture ratio of H-2:CO was 3:7 and 9: 1, respectively. In case of 100% carbon monoxide (CO) without hydrogen (H-2) addition, the weight of carbon increased, but CNTs were not observed. The CNTs were formed when the contents of H-2 reaches at least 10%, and their structures became more distinct with an increase of H-2 addition, and then the shapes of CNTs were more thin and straight. When the contents of H-2 was 80% (H-2:CO = 8:2), the shapes and growth of CNTs showed an optimal condition. On the other hand, when the contents of H-2 was higher than the critical value, the shapes of CNTs became worse due to transition into inactive surface of catalyst. It was found that H-2 played a major role in the shapes and structures of CNTs.