The relations between the enhanced low-dose-rate Effects (ELDREs) of radiation-sensitive field effect transistors (RADFETs) developed by the National Micro-electronics Research Centre (NMRC) and the vacant oxide trap densities in SiO(2) were found. The ELDREs of the NMRC RADFETs occured only when the vacant oxide trap density was high. To ensure these relations, we found the distribution of radiation-induced electron-hole pairs in SiO(2) by using a MCNPX(Monte Carlo N-Particle eXtended) simulation and we performed other radiation tests for the NMRC RADFETs and for the commercial 3N163 FETs developed by Linear Systems. With the above results, we also present some possible reasons previous research could not find the ELDREs.