One- and two-dimensional diffraction gratings were fabricated on commercial polyethylene terephthalate (PET) film by two beam direct laser interference patterning (DLIP) method using a pulse laser (10 ns) with a wavelength of 266 nm. The relative diffraction efficiency of the structured specimens (at 633 nm) in transmission mode was measured and correlated with the topological characteristics of the structured PET substrates. The obtained diffraction gratings made by DLIP show better or similar diffraction efficiencies when compared with the reported efficiency of gratings made by other methods (up to 42.5% for 1 μm arrays), and can be fabricated on large areas in very short processing times (up to 18 cm²/s). POLYM. ENG. SCI., 52:1903-1908, 2012. © 2012 Society of Plastics Engineers