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 N = (2, 0) and (1, 1) little strings as well as 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.where (c R − c L ) is the 2d gravitational anomaly. Similarly, the gauge non-invariant action S (2) must match the 2d chiral anomaly under the U (1) gauge transformation, i.e., δ Φ z = 0 and δ A z = d z . Recall that the 2d chiral anomaly ∆ is encoded in the 4-form anomaly polynomial I 4 by the descent formalism, such thatwhere the sum is taken over all background U (1) gauge fields. Dimensionally reducing it on S 1 t ,