Tertiary recrystallized grain-oriented silicon steel sheets with thickness from 80pm to 20pm are developed. Such ultra thin sheets are produced with appropriate cold-rolling and annealing. The magnetic induction B8 of the sheet is above 1.95T and the coercive force Hc is below 7A/m. ac iron loss of the sheet depends on the sheet thickness and is minimized for 30μm thick sheet having W13/50=0.33W/kg. This value is about 3/5 of that of the best grade conventional silicon steel sheet. The loss is reduced further by W13/50=O.17W/kg when the tensile stress of 4kg/mm2 is applied to the sheet. This loss is comparable to that of Fe-Si-B amorphous sheet. Moreover, grain-oriented sheets of 5pm thick can be obtained by applying cold-rolling to the tertiary recrystallized sheets with the thickness of 20pm and then annealing them.