The deformability of a Fe-Al-Si alloy (Sendust) which has been considered very difficult to deform plastically was studied by the bending test and the sheath forging test at elevated temperatures of 800 to 1000°C using a cast ingot of an alloy containing about 6.44%Al, 10.32%Si, and 0.68%Ti. As for the magnetic properties, the magnetic hysteresis loop and effective permeability were examined for the alloy deformed by hot sheath forging. The results obtained are as follows: (1) By bending tests at elevated temperatures and under high hydrostatic pressures, the brittle to ductile transition temperature at atomospheric pressure and the transition pressure at room temperature were observed to be in the temperature range of 800 to 850°C and at a pressure higher than 12000 kg/cm², respectively. (2) Using a thick-walled sheath of mild steel, the alloy was successfully deformed even at 800°C by hot forging. In the present study, the reduction in thickness more than 70% could be given on the alloy by forging at 1000°C. (3) The bending load at fracture of the alloy deformed about 72% in thickness was obtained to be about two times larger than that for the as-cast alloy. (4) The magnetic properties of the forged alloy were observed to be inferior to that of the as-cast alloy especially in the low frequency range but they were improved almost to the same order of magnitude as the cast alloy by annealing at 1000°C.