The relative energy dependence of the 235U fission cross section has been measured with respect to the (n, p) scattering reaction for neutron energies from 3 to 20 MeV. The Lawrence Livermore Laboratory Linac provided a pulsed source of neutrons and energies were measured by neutron time of flight. The flux monitor consisted of an annular polyethylene proton radiator with a shielded recoil detector. The total error in the relative 235U(n, f) cross section is ±1% from 3 to 7 MeV, ±2% at 14 MeV, and ±6% at 20 MeV.