CNG-gasoline blended fuel has been considered as a potential choice of alternative fuel for spark ignition (SI) engines specially turbocharged one to utilize advantages of both fuels. In this study methane (main component of CNG) is added in two volumetric fractions of 30% and 70% to iso-octane (representative fuel of gasoline). Spherical flames are experimentally investigated at initial temperature of 363 K and pressure of 1 bar in constant volume chamber. Classical schlieren technique used to characterize the combustion of blended and base fuels. Unstretched flame propagation speed and Markstein lengths are obtained via nonlinear methodology in different equivalence ratio. The laminar burning velocities, Markstein number and burning flux of blended fuel are then extracted and compared with base fuels. The results show that addition of methane to iso-octane increases the unstretched propagation speed in lean region but decrease the unstretched propagation speed in rich region. Blended fuels response to stretch is a linear combination of base fuels in lean region (empty set > 1); but methane has more Markstein length and Markstein number than the other fuels in rich region (empty set > 1). Laminar burning velocity and flame stability of 30% methane blended fuel is close to iso-octane. Laminar burning velocity of 70% methane blended fuel is higher than base fuel at empty set = 0.9. The tests also repeated for CNG/isooctane blended fuel and similar trend obtained for laminar burning velocities. (c) 2014 Published by Elsevier Ltd.