  We study the compositeness of unstable hadrons which lie near the two-hadron
threshold. In the framework of the effective field theory, we derive the
relation between the compositeness of stable bound states with observables. We
then extend this relation for the quasi-bound states with finite decay width. A
prescription to interpret the complex value of the compositeness is presented.
With this method, we show that Lambda(1405) is dominated by the Kbar N
composite component and a_0(980) have only small Kbar K fraction.
