The work aims to study the effects of different H2/Ar flow rates on morphology and structure of nanocrystalline diamond. Better nanocrystalline diamond (NCD) films were prepared by MPECVD method, and the morphology, structure and grain size of diamond films were tested by SEM and XRD. In addition, the variation trend of diamond D peak and nano diamond peak (TPA) was also analyzed by using Raman. Provided with Q(H2):Q(Ar)=50:49, the prepared diamond grains fell within submicron category, the average grain size was 250 nm, surface smoothness was poor, stacking fault occurred while the diamond characteristic peak (D peak) reached the maximum with fastest growth rate up to 125 nm/h; provided with Q(H2):Q(Ar)=10:89, surface smoothness was high, second nucleating phenomenon was obvious and the average grain size was 20 nm; provided with Q(H2):Q(Ar)=0, the grains changed from nano to super nano category, second nucleating phenomenon was more obvious, the surface roughness was higher and the average grain size was 3 nm. In addition, the Raman test showed that the diamond characteristic peak intensity decreased with the decrease of flow rate of H2/Ar while that of nano diamond increased with the decrease of the H2/Ar flow ratio. With the increase of H2/Ar flow rate, the surface roughness of diamond was gradually getting worse, and the surface roughness was gradually increasing. Meanwhile, grain size and growth rate of the diamond reached the maximum provided with Q(H2):Q(Ar)=50:49.