The effect of H2 gas during the carbon nanotubes (CNTs) synthesis with CO-H2 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 H2:CO was 3:7 and 9:1, respectively. In case of 100% carbon monoxide (CO) without hydrogen (H2) addition, the weight of carbon increased, but CNTs were not observed. The CNTs were formed when the contents of H2 reaches at least 10%, and their structures became more distinct with an increase of H2 addition, and then the shapes of CNTs were more thin and straight. When the contents of H2 was 80% (H2:CO = 8:2), the shapes and growth of CNTs showed an optimal condition. On the other hand, when the contents of H2 was higher than the critical value, the shapes of CNTs became worse due to transition into inactive surface of catalyst. It was found that H2 played a major role in the shapes and structures of CNTs.