Laminar counterflow diffusion flames provide useful information about basic properties of non-premixed combustion. A lot of studies have been performed on H2/O2 counterflow. Most of previous studies include undiluted or diluted systems of hydrogen (with CO2 or N2) against an air flow under a variety of conditions such as strain rates, inlet temperature and pressure. The main purpose of this research is to investigate the extinction limits and also to see how the temperature changes against the strain rate. The effect of Ar and He gases as diluents on combustion and extinction limits of H2/O2 is also investigated. Numerical simulations have been done with the OPPDIF code from CHEMKIN software. The obtained numerical results are compared with the others results and show the accuracy of numerical solution. Increasing the diluent percent causes decreasing of the maximum temperature and also the maximum mole fraction of active radicals of H and OH. Decreasing of producing H radical causes the fuel be slightly influenced by this species and thus the fuel diffuses more to oxide side. Increasing the diluent percent also causes decreasing of blow off and quenching limits in hydrogen combustion and accordingly narrows the flammability; But the increasing of inlet temperature flows and ambient pressure extend the flammability limits.