Flame propagation characteristics of natural gas-hydrogen-air mixtures were studied in a constant volume combustion bomb at room temperature and low initial pressures. The laminar burning velocities, the mass burning fluxes, and the Markstein number were obtained at various hydrogen fractions and equivalence ratios. The results show that the unstretched laminar burning velocity and the mass burning flux increase with the increase of hydrogen fraction in a natural gas-hydrogen blend. Flame stability tends to increase with the increase of equivalence ratio. For a specific equivalence ratio, the Markstein number decreases with the increase of hydrogen fraction, leading to an increase of thermodiffusively generated flame instability. Flaw and protruding or even honeycomb-like structure occur at the flame front area under lean mixture combustion while rich mixture combustion maintains a smooth flame front surface. Lean mixture increases flame instability while rich mixture increases flame stability. At a large hydrogen fraction (larger than 80%), rho(u)/rho(b) is increased and flame thickness is decreased with the increase of hydrogen fraction, leading to the increase of flame instability, and easily forms a cellular surface at flame front.