Laser spark ignition of methane diffusion flames, in the stagnation region in counter-flow configuration, is studied with a range of transport properties achieved by using different levels of inert dilution with helium and argon as diluents in the fuel stream. The effects of transport properties on ignition strain rate and ignition delay time are investigated and possible effects of effective Lewis number (Le(e)) are also discussed and compared with those for flame extinction. The following results are obtained: (1) The role of Le(e) on the critical global strain rate, beyond which ignition is not possible, is qualitatively similar to that on the extinction strain rate. That is, with the same level of dilution, the inert diluent with smaller Le(e) (therefore relatively smaller heat loss rate) yields larger critical global strain rate. (2) For successful ignition, the ignition delay time decreases with increasing level of helium dilution, while the opposite is true with argon dilution - the experimental results correlated with the calculated results of the diffusion times with these two inert diluents, suggesting that the successful ignition and the ignition delay time is limited by the thermal diffusion time across the flammable layer in the counterflow region, rather than resulting from the Lewis number effect. (C) 2018 Elsevier Ltd. All rights reserved.