The ferromagnetic semiconductor La2NiMnO6 (LNMO) has recently received much attention due to its high Curie temperature (TC ~ 280 K), which is close to room temperature. We prepared single-phase LNMO polycrystalline samples and investigated the temperature- and field-dependent magnetic behaviors of bulk LNMO. Between TC and T* = 300 K, we observed upward and downward deviations from the Curie—Weiss law for high and low magnetic fields, respectively. From the electron spin resonance results, we can exclude the existence of the Griffiths phase. On the contrary, our results indicate that the abnormal magnetic behaviors might be induced by antisite phase boundaries with antiferromagnetic interaction.