The low energy transmission grating spectrometer (LETGS) on board the Advanced X-ray Astrophysics Facility provides high resolution dispersive spectroscopy between 70 eV and more than 7 keV. The LETG contains 180 grating modules, each equipped with 3 grating facets. The freestanding gold gratings have 1008 lines per mm. Early 1997, the AXAF telescope underwent extended calibrations in the long beam X-Ray Calibration Facility at the NASA/Marshall Space Flight Center. As part of the telescope, also the performance of the LETGS with respect of spectral resolving power and effective area was measured. At more than 50 individual energies we have measured the grating efficiency or the effective area of the spectrometer, respectively. All these energies were chosen in order to cover the numerous spectral features due to absorption edges of filters, detector coatings, mirror reflectivities, and grating efficiency variations. Although preliminary, the performance of the gratings is close to the predictions made on the basis of subassembly measurements of individual grating elements. In particular, the first order efficiency is about 15% (both sides including vignetting effects) outside the energy regime of partial transparency of the grating wires; inside the efficiency gains from constructive interference effects. Both first diffraction orders are symmetric within less than 1%. The second order is suppressed by a factor of about 200 relative to the first order.