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The distribution of He in η-Fe2C has been studied by first-principles calculations. The formation energies of interstitial He and substitutional He (replacing Fe) are 3.76 eV and 3.49 eV, respectively, which are remarkably smaller than those in bcc Fe, indicating that He is more soluble in η-Fe2C than in bcc Fe. The binding potencies of both a substitutional-interstitial He pair (1.28 eV) and a substitutional-substitutional He pair (0.76 eV) are significantly weaker than those in bcc Fe. The binding energy between the two He atoms in an interstitial–interstitial He pair (0.31 eV) is the same as that in bcc Fe, but the diffusion barrier of interstitial He (0.35 eV) is much larger than that in bcc Fe, suggesting that it is more difficult for the interstitial He atom to agglomerate in η-Fe2C than in bcc Fe. Thus, self-trapping of He in η-Fe2C is less powerful than that in bcc Fe. As a consequence, small and dense η-Fe2C particles in ferritic steels might serve as scattered trapping centers for He, slow down He bubble growth at the initial stage, and make the steel more swelling resistant. [ABSTRACT FROM AUTHOR]