  We propose a method to obtain physical quantities in the theta vacuum from
those at fixed topology, which are different by finite size effects. Extending
the work by Brower et al., we derive the formula to estimate these finite size
corrections for arbitrary correlators in terms of the topological
susceptibility and the theta dependence. Applying this formula, we show that
topological susceptibility can be measured through two-point functions of
pseudoscalar operator.
