An experimental investigation was conducted in an optical mono-cylinder Spark-Ignition engine in order to explore the influence of the fuel and of the dilution rate on the initial stages of flame propagation. Images of flame radiation were acquired through the transparent piston crown with a high speed CMOS camera operating at 6000 frames/second. Experiments were performed under stoichiometric and lean conditions (0.8 of equivalence ratio), and two engine speeds (1200 rpm and 2000 rpm). The spark ignition timing was set at 30 (iso-octane) and 25 (methane) crank angle degrees before top dead center. Image acquisition was synchronized with in-cylinder pressure to allow simultaneous evaluation of the Indicated Mean Effective Pressure (IMEP) and of the heat release rate. Image post-processing was performed to obtain the temporal evolution of the projected flame area. A flame initiation criterion was defined and a flame initiation delay, based on the optical investigation, was determined for all mixtures and conditions. The effects of the fuel (pure iso-octane, methane and hydrogen blended mixtures) on the initial stages of flame propagation were studied. The effect of dilution, here simulated only by the addition of nitrogen, was also investigated. Results show that the initial flame development duration increases with the dilution rate and decreases with hydrogen addition in the air-fuel mixture. The standard deviation of the crank angle corresponding to a defined projected enflamed area (50% of the total visible area) varies with fuel composition and increases with dilution. The initial stages of flame propagation were found to correlate very well with the laminar burning velocities obtained at atmospheric pressure and ambient temperature. The positive effect of hydrogen addition is more pronounced under diluted and lean conditions compared to stoichiometric conditions. The addition of hydrogen to avoid the drawbacks of dilution by burned gases (EGR or IGR) and of lean conditions can be one solution to provide sufficiently stable combustion and therefore low thermal NOx formation in S.I. engines.