Sodium boiling experiments were conducted using the out-of-pile sodium loop with 91 mimic fuel pins subassembly in order to establish the acoustic detection method of sodium boiling in an FBR reactor core. The following results were obtained concerning the characteristic of sodium boiling sound, the method of signal processing and the void effect. (1) Acoustic pressure by sodium boiling increases according to the boiling level, and the maximum acoustic pressure reaches about 1, 000 Pa. (2) Boiling appears with a constant period; the period becomes longer with the increase of satura-tion temperature, to about 0.7s at last. (3) Acoustic spectrum detected by an in-sodium sensor has mainly lower frequency components; and by an on-wrapper tube sensor, mainly higher frequency components. (4) Smoothing processing of acoustic signals is necessary for the cross-correlation treatment in order to obtain the time lag between the signals of two sensors. Also, informations on the acoustic pressure due to fission gas release and its diminishing effect on boiling acoustics were obtained through the mimic test with Ar gas injection.