  We show that the cosmological constant at late time places a bound on the
entropy of microwave background radiation deposited in the future event horizon
of a given observer, $S\leq S_{\Lambda_0}^{3/4}$. This bound is independent of
the energy scale of reheating and the FRW evolution after reheating. We also
discuss why the entropy of microwave background in our observable universe has
its present value.
