  We study the elliptic genera of 6d strings based on their modular properties.
They are weak Jacobi forms of weight 0, whose indices are determined from the
2d chiral anomalies. We propose the ansatz for the elliptic genera which
reflects the analytic structure of instanton partition functions. Given a
finite amount of initial BPS data, we completely determine the elliptic genera
of 6d strings in various 6d SCFTs. We also apply our ansatz to study
$\mathcal{N}=(2,0)$ and $(1,1)$ little strings as well as $\mathcal{N}=(1,0)$
heterotic little strings, for which T-duality of little string theories
supplies a sufficient number of initial BPS data. The anomaly polynomials of 6d
little strings are worked out, which is needed for the elliptic genera
bootstrap. In some little string theories, the elliptic genera must have the
extra contributions from the Coulomb branch, which correspond to the additional
zero modes for the full strings. The modified ansatze for such elliptic genera
are also discussed.
