We present a velocity structure of the mantle discontinuities beneath the United States from a full waveform inversion of broadband seismic data. The data consist of 10 moderate earthquakes in the western United States that were recorded on 13 broadband seismometers across the United States operated by the U. S. National Seismic Network. A total of 36 seismograms with distance ranging from 1150 to 2700 km were used. A full waveform inversion based on two-dimensional modeling and one-dimensional inversion methods using a genetic algorithm was adopted. The inversion method is objective, provides error bounds on the model, and does not require any a priori information about these discontinuities. We find that the upper discontinuity is ~10 km thick, from 395 to 405 km, where the velocity increases from 8.80 to 9.15 km s-1 with a velocity gradient of 0.035 s-1. The lower discontinuity is ~5 km thick, occurring over a depth range of 650--655 km, where the velocity increases from 10.20 to 10.70 km s-1. Our results are in agreement with previous studies of the lower discontinuity but do not agree for a sharp (<5 km) upper mantle discontinuity. ¿ 2001 American Geophysical Union