Deep level transient spectroscopy (DLTS) and lifetime measurements were carried out on silicon p+nn+ diodes before and after irradiation with fast neutrons at room temperature with fluences of 5.5 × 1011 and 1.0 × 1012 n/cm2. In the non‐irradiated samples the observed levels satisfactorily explained the measured data of recombination and generation lifetimes in the frame of the Shockley‐Hall‐Read (SHR) theory. After irradiation the lifetime data showed discrepancies with the parameters of the existing levels, which was explained by the cluster structure of the neutron damage defects.