  We establish a relation between Hahn spin-echo of a spin-$\frac 1 2 $
particle and quantum phase transition in a spin-chain, which couples to the
particle. The Hahn echo is calculated and discussed at zero as well as at
finite temperatures. On the example of XY model, we show that the critical
points of the chain are marked by the extremal values in the Hahn echo, and
influence the Hahn echo in surprising high temperature. An explanation for the
relation between the echo and criticality is also presented.
