A model of turbulent sub-grid scale flame speed for premixed combustion is proposed and tested in Large Eddy Simulation (LES). The model is based on writing the unresolved flame surface density in terms of a general power-law expression that involves an inner cutoff scale. This scale is derived from an equilibrium assumption of flame-surface production and destruction. The flame-surface production term is modeled using a parameterization of the effective flame stretch, obtained from a spectral superposition of earlier DNS results of single vortex-flame interactions [8]. The model is implemented in a LES combustion code in the context of Thickened Flame LES (TF-LES) using an empirically chosen (non-dynamic) value for the power-law exponent. Three-dimensional simulations of premixed flame embedded in a time decaying isotropic turbulent flow are performed in several different parameter ranges. Comparisons between direct numerical simulation (DNS) and LES using different resolutions and thickness factors show that the LES reproduces the total reaction rate of the DNS quite well and independently on the thickness factor, resolution, and sub-grid scale model used for the turbulent eddy viscosity. Comparisons between the predicted overall turbulent flame speed s(T) as function of the r.m.s. velocity and experimental data show good agreement over a significant range of parameters.