We have determined the dependence of exchange coupling an the thickness of both an antiferromagnet (AF) layer (t(AF)) and a ferromagnet (FM) layer (t(FM)) in Fe50Mn50/Ni81Fe19 bilayers. The thickness of FeMn layer varies from 0.5 to 32 nm With the fixed NiFe layer of 30 nm, whereas that of NiFe layer varies from 3.5 to 45 nm With the uniform FeMn layer fixed at 10 nm. For the dependence on t(FM), the exchange field H-ex varies as 1/t(FM)(0.9) whereas the dependence of H-ex on t(AF) displays a behaviour of power law as 1/t(AF)(0.3) at both 80 and 300 K for t(AF) > 3 nm. Temperature dependence of H-ex shows that blocking temperature (T-B) decreases as t(AF) decreases far thinner FeMn layers.