Field-effect transistors (FETs) fabricated on hydrogen-terminated diamond surface have been heavily irradiated with 14.8-MeV neutrons in order to evaluate their possible application in very high neutron fluence environments. The dc performance of the diamond-based FETs, such as drain saturation current and maximum transconductance, has been studied as a function of a 14.8-MeV neutron fluence up to 1014 n/cm2, delivered in five steps. The effects on electrical properties of H-terminated diamond surface have also been investigated during the neutron irradiation experiments. The Hall parameters, i.e., sheet hole concentration, hole mobility, and sheet resistance, were monitored before and after each irradiation. The performance remains stable during all the neutron fluence steps, thus assessing a remarkable radiation hardness of diamond-based devices. To the best of our knowledge, this is the first published data on 14-MeV neutron tolerance of diamond FET devices. © 2016 IEEE.