This paper is conceed with the problem of scattering of time-harmonic electromagnetic waves from penetrable diffraction gratings in the 2D polarization case.We propose a new,weakly singular,integral equation formulation for the scattering problem which is proved to be uniquely solvable.A main feature of the new integral equation formulation is that it avoids the computation of the normal derivative of double-layer potentials which is difficult and time consuming.A fast numerical algorithm is also developed for the scattering problem,based on the NystrSm method for the new integral equation.Numerical examples are also shown to illustrate the applicability of the new integral equation formulation.