Abstract
We have studied the formation process of ferroelastic domain in Rochelle salt by means of light deflection. Although the deflection is thought to occur at domain boundaries which exist in the ferroelastic phase, the deflected light is observed even 0.08°C above the phase transition temperature. We consider the phenomenological model which introduces the intrinsic stress distribution inside the crystal and show that the distribution produces embryonic domain structure which grows into the distinctive ferroelastic domains through the instability caused by the phase transition.