Effects of turbulent flame stretch on mean local laminar burning velocity of flamelets, (u) over bar (n), were investigated experimentally in an explosion vessel at normal temperature and pressure. In this context, the wrinkling, A(t)/A(l), and the burning velocity, u(t), of turbulent flames were measured simulataneously. With the flamelet assumption the mean local laminar burning velocity of flamelets, (u) over bar (n) center dot (A(t)/A(l))(-1), was calculated for different turbulence intensities. The results were compared to the influence of stretch on spherically expanding laminar flames. For spherically expanding laminar flames the stretched laminar burning velocity, u(n,) varied linearly with the Karlovitz spherically expanding laminar flames the stretched laminar burning velocity, u(n), varied linearly with the Karlovitz stretch factor, yielding Markstein numbers that depend on the mixture composition. Six different mixtures with positive and negative Markstein numbers were investigated. The measurements of the mean local laminar burning velocity of turbulent flamelets were used to derive an efficiency parameter, l, which reflects the impact of the Markstein number and turbulent flame stretch-expressed by the turbulent Karlovitz stretch factor-on the local intensity and the reduction can be correlated to unsteady effects. (C) 2008 The Combustion Institute. Published by Elsevier Inc. All right reserved.