This work deals with the characterization of the Microwave Plasma Assisted Chemical Vapour Deposition (MPACVD) process suitable for nanocrystalline diamond growth. The microwave system used in this study was a Bell Jar reactor and the discharge was ignited in an Ar/H2/CH4 gas mixture. We were especially interested in the investigation of the effect of the simultaneous increase of the microwave power and H2 concentration. Diamond films were characterized by appropriate surface diagnostic techniques. The plasma was analysed through modelling and spectroscopic diagnostics. Results showed in particular that the increase of H2 amount and microwave power strongly influences the diamond film nanostructure. Besides, the high values measured for gas temperature were in good agreement with those predicted by the model. Eventually, the model showed that the relatively high gas temperature in Ar/H2/CH4 discharges was responsible of a strong thermal dissociation of H2.