To find the possibility in growing carbon nanotubes (CNTs) used as field emitters, different N2 proportions were mixed with H2/CH4 ingrowing CNTs. It was found that N2 has significant impact on the growth, diameter and cleanness of CNTs. CNTs with different diameters weregrown on the same nickel catalyst particle size with different N2 proportions. The suppressing CNT growth behavior was observed with 30% N2 inprocess gas. The CNTs grown from 47 and 57% N2 possess smaller diameter and less carbonaceous particles than those grown from 0% N2. Theabove phenomena are explained by NH3 and nickel nitride formation during CNT growth claims proposed by others, together with the authors'proposal: the CNTs are grown selectively on nickel portion instead of the whole catalyst particle constructed by nickel mixed with nickel nitrideduring CNT growth. This work brings the potential to produce slim and clean CNTs with controlling N2/H2/CH4 proportion, which is arequirement for field emitter application.