The flammability and explosion characteristics of natural gas (NG) and air mixtures at high pressure and high temperature are of great concerns for air injection IOR (improved oil recovery) process in oilfield and for natural gas combustion engines. In this study, an experimental setup is built up to measure the explosion limits (flammability limits) of methane and natural gas at elevated pressures and temperatures up to 20 MPa and 100 degrees C. The influences of pressure, temperature, and ignition energy and gas composition on the flammability limits, minimum oxygen content required for explosion (MinOC) and explosion energy have been investigated for methane and typical natural gas mixtures with air. High pressure and high temperature flammability limit models for natural gas-air mixtures have been proposed based on the experimental data. The experimental results show that the flammability limits of natural gas can be significantly extended at high temperature and high pressure, indicating a good flammability for combustion applications but high explosion risks in terms of the safety for the oilfield air injection process. At 100 degrees C and 20 MPa, the explosion limit range measured for methane in air is extended to 2.87%-64.40%vol, in contrast to the range of 4.95%-15.51%vol at the normal pressure and temperature (0.1 MPa and 25 degrees C), and the corresponding minimum oxygen content required for explosion can be reduced to as low as 5.74% from around 10% at the atmospheric conditions. The variation of the upper flammability limit (UFL) is more sensitive than that of the lower flammability limit (LFL) as pressure and temperature increase. Electrically heated tungsten wires were applied for the ignition, and it has been found that high ignition energy is required to ignite the gas mixtures at high pressure conditions. The peak explosion pressure (or explosion energy) is mainly depended on the composition of the flammable gas and oxygen, while it is slightly reduced at high temperature conditions and gradually increases with the initial pressure. Explosions with incomplete combustion reaction can occur at near UFL points with enormous amount of carbon black formed due to lack of oxygen, while the explosion energy can be significantly reduced at near LFL points because of lack of flammable gases. The increase of risk index at high pressure and high temperature indicates a larger explosion risk and safety concern.