Effect of strain rate on flame extinction is numerically investigated in downstream interaction among lean (rich) and lean (rich) premixed as well as partially premixed (50% H-2 + 50% CO)-air flames. The strain rate varies from 30 to 4671 s(-1) until interacting flames cannot be sustained anymore. Flame stability diagrams mapping lower and upper limit fuel concentrations for flame extinction as a function of strain rate are explored. It is shown that significant increase of strain rate makes even the upper rich extinction boundary be slanted due to strong chemical interaction. The lower extinction boundary can be also extended to rich fuel concentrations over the stoichiometric mixture condition when strain rate significantly increases. The lower and upper extinction boundaries, mainly caused by the conductive heat loss from the stronger flame to ambience, become narrower and narrower in increasing strain rate. The results also show that the extinction boundaries with positive flame speed are extended and then reduced in increasing strain rate, thereby leading to an island of extinction boundary and subsequently being changed into a point on the symmetric fuel concentration line. The detailed explanations on those flame extinction characteristics in the stability diagrams are made through analysis of the flame structures in increase of strain rate. The mechanism of flame extinction is also proposed and discussed in detail. (C) 2012 Elsevier Ltd. All rights reserved.