If the thickness of pancake austenite grains becomes as small as 1mum through heavy compression, the size of ferrite grains formed at austenite grain boundaries is expected to be as small as the thickness. Specimens with chemical composition of 0.16C-0.4Si-1.4Mn were compressed by 75% in reduction at 1023K and then cooled at 10K/s. Various pancake austenite grains with the thickness between 300mum and below 1mum were obtained through the compression in a specimen. The relationship between the thickness of the pancake austenite grain and the ferrite grain size transformed from the austenite grains was studied. The ferrite grain size is dominated by the thickness of the pancake austenite grain rather than by strain or prior austenite grain size when the thickness is less than 10mum. The smaller the thickness, the smaller the ferrite grain size is. However, when the thickness reaches less than 2mum, the ferrite grain size becomes independent of the thickness.