The morphological changes and the coarsening kinetics of B2 particles in A2 matrix in the elastically constrained Fe-16at%Al-18at%Co alloy were investigated by means of transmission electron microscopy (TEM). The mean particle radius increases in proportion to the cube root of ageing time at early stage, then the coarsening of B2 particle decelerate at late stage of aging. And it clarified the decelerated coarsening in Fe based alloy also is related to the change in PSD and is caused by the elastically strained effect.