The formation of austenite from a coarse grained ferrite-pearlite microstructure during intercritical annealing was studied. According to the variation of the microhardness values in ferrite and austenite with intercritical annealing time, austenite formation was classified into four stages: (a) austenite growth into pearlite at a slower rate than the dissolution rate of pearlite, (b) subsequent growth of austenite into pearlite and formation of thin film type austenite at ferrite grain boundaries, (c) growth of austenite into ferrite, (d) equilibrium of ferrite and austenite. In particular, plate-like austenite was observed. The experimental results indicated that the growth process of this type austenite was controlled by carbon diffusion in austenite.