The LaBr3(Ce) detector has attracted much attention in recent years because of its superior character-istics compared with other scintillating materials in terms of resolution and efficiency. However, it has a relatively high intrinsic background radiation because of the naturally occurring radioisotopes in lanthanum, actinium, and their daughter nuclei. This limits its applications in low-counting rate experiments. In this study, we identified the radioac-tive isotopes in the φ3″ × 3″ Saint-Gobain B380 detector by a coincidence measurement using a Clover detector in a low-background shielding system. Moreover, we carried out a Geant4 simulation of the experimental spectra to evaluate the activities of the main inteal radiation com-ponents. The total activity of the background radiation of B380 is determined to be 1.523 (34) Bq/cm3. The main sources include 138La at 1.428 (34) Bq/cm3, 207Tl at 0.0135 (13) Bq/cm3, 211Bi at 0.0136 (15) Bq/cm3, 215Po at 0.0135 (3) Bq/cm3, 219Rn at 0.0125 (12) Bq/cm3, 223Fr at 0.0019(11) Bq/cm3, 223Ra at 0.0127 (10) Bq/cm3, 227Th at 0.0158 (22) Bq/cm3, and 227Ac at 0.0135 (13) Bq/cm3. Of these, the activities of 207Tl, 211Po, 215Po, 223Fr, and 227Ac are deduced for the first time from the secular equilibrium established in the decay chain of 227Ac.