Impulsive stimulated Raman scattering (ISRS) was applied to some ferroelectric crystals to analyze the vibration of the crystal lattice in time domain. For significant phonon excitation to occur,the excitation pulse duration τ<2 π/ ω α. Using Ti∶Sapphire regenerative amplifier system as a light source, phono polariton oscillatory modes in LiTaO 3 could be observed. The method was applied to KH 2PO 4. Only relaxing was observed within 50 K from the ferroelectric phase transition point. The relaxation time constant τ 0 is 1.33×10 -13 s in the wave vector q=6848 cm -1 . It shows a relaxational character of the ferroelectric phase transition KH 2PO 4.