Phonon energy of rubidium chloride was measured at ambient pressure and at 4.9 kbar which is just below the structural phase transition pressure from B1 to B2 phase. The energies of the [0 0 1]LA mode at 4.9 kbar increased from those at ambient pressure. On the other hand, the energies of the [0 0 1]TA mode slightly decreased. The energy of the [1 1 1]TA mode decreased near the zone boundary, although the energy of this mode increased in the range below 0.4 in q/q m a x .