Synthesis process of graphene nano-flakes produced in radio-frequency inductively coupled plasma system by axially injecting CH4 into Ar-H-2 plasma is investigated at different process conditions such as plate power, pressure and CH4 flow rate. Optical emission characteristics of Ar, Ar-H-2, Ar-H-2-CH4 and Ar-H-2-C2H2 plasmas are sequentially studied to understand different dissociation channels of CH4. Graphene nano-flakes are formed by condensation of C-2 species which are generated owing to CH4 decomposition caused by electron impact, dehydrogenation and energy transfer from excited state Ar atoms to C2H2 and/or C2H produced during dissociation. Maximum production rate achieved is similar to 10g/h at plate power of 22kW, pressure of 70kPa and CH4 flow rate of 1.5 slpm at which intensity ratio of D and G band of Raman spectrum is 0.25 in spite of having contribution from edges of nano-flakes to D band intensity indicating good quality. Effect of decrease in cooling rate at the location of nucleation at higher CH4 flow rates >= 2slpm increases average number of graphene layers (#L) which is however reduced with increase in plate power i.e. from #L of 22 to 13 with increase in plate power from 15 to 22kW at pressure of 70kPa and CH4 flow rate of 2slpm.