This paper presents data on the lower and upper flammability limits of hydrogen air mixtures at elevated temperature and pressure. A 5-L explosion vessel, an ignition system, and a transient pressure measurement sub-system were used in this study. Through a series of experiments carried out, the lower and upper flammability limits of hydrogen air mixtures at different initial pressures and temperatures have been studied and the influence of initial temperature and pressure on the lower and upper flammability limits of hydrogen air mixtures has been analysed and discussed. It was found that the decrement of the LFLs of hydrogen air with the initial temperature from 21 to 90 degrees C at the initial pressure of 0.1 MPa is less than 1%, the decrement of the LFLs with the initial temperature from 21 to 90 degrees C at 0.2 MPa is less than 1%, the decrement of the LFLs with the initial temperature from 21 to 90 Cat 0.3 MPa is less than 0.66%, and the decrement of the LFLs with the initial temperature from 21 to 90 degrees C at 0.4 MPa is less than 0.25%. The lower flammability limits of hydrogen air mixtures at the pressures of 0.1 and 0.4 MPa are 4 and 1.25%(V/V), respectively. The upper flammability limits of the hydrogen air mixtures increase with the initial pressure and temperature. The upper flammability limit of the hydrogen air mixtures at 90 degrees C and 0.4 MPa reaches 93%(V/V) which is much higher than that (76%(V/V)) at 21 degrees C and 0.1 MPa. Copyright 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.