The influence of In diffusion on resistance stability of AuIn2thin‐film resistors was studied and the mechanism for resistance change was clarified. When resistors with In concentration below that of AuIn2were in contact with Pb‐In‐Au alloy interconnection lines, In diffused from the Pb alloy into the resistors. This diffusion caused grain growth in the In‐diffused zone, in addition to a phase change from an AuIn and AuIn2mixture to AuIn2single phase. The In‐diffusion‐induced grain growth decreased film resistivity because resistivity is mainly determined by grain‐boundary scattering. When the resistor was preannealed before being connected to interconnection lines to saturate grain growth, excellent resistance stability was obtained even though In concentration shifted by ∼2 wt. % below AuIn2stoichiometry.