This paper reports both the lower and upper flammability limits of hydrogen-enriched natural gas with hydrogen fractions of 20%, 40%, 60% and 80% respectively as well as these of natural gas and hydrogen, measured by using a constant volume combustion chamber together with a high-speed schlieren photographic system. Based on investigating pressure rise history inside the combustion chamber as well as flame photos, the effect of hydrogen enrichment on the flammability characteristics is discussed. Our experimental results show that the flammability limits of methane-hydrogen mixtures can be used for hydrogen-enriched natural gas as long as their hydrogen fractions are the same. In this paper, the flammability data of methane-hydrogen mixtures available in the literature are reviewed. Correlations for both the lower and upper flammability limits of methane-hydrogen mixtures are summarized.