Natural gas-hydrogen-air premixed combustion was studied in a constant volume bomb over wide ranges of equivalence ratios and hydrogen fractions and two initial pressures. A two-zone model was used to calculate heat release rate and combustion durations based on the pressure data. The study shows that, with the increase of hydrogen fraction in the mixture, the normalized mass burning rate increases while the flame development duration and the total combustion duration decrease with the increase of the hydrogen fraction in natural gas-hydrogen blends over various equivalence ratios. A small difference in maximum pressure for various hydrogen fractions is presented at the equivalence ratios near the stoichiometric equivalence ratio. The maximum pressure increases with the increase of the hydrogen fraction in the mixture for lean mixture combustion. Short combustion duration is presented over wide ranges of equivalence ratios with increasing hydrogen fractions in the mixture for rich mixture combustion. The difference in flame development duration for mixtures with various hydrogen fractions increases with a decreasing equivalence ratio for lean mixture combustion and increases with an increasing equivalence ratio for rich mixture combustion. The ratio of the flame development duration to the total combustion duration increases with an increasing hydrogen fraction in the mixture, and this reveals the fact that hydrogen addition has a larger influence on the total combustion duration rather than on the flame development duration.