Application of an electronic packaging material is always limited by its established coefficient of thermal expansivity (CTE) and poor plastic working capacity. Here, we explored that a ductile high-entropy alloy (HEA) could achieve adjustable CTE values via decomposing at 500 °C. The carbon-doped equiatomic FeCoCrNiMn HEAs (0.8, 1.0, and 1.3 at% C) decomposed into B2, L1 0 , M 23 C 6 and σ phases during annealing. The adjustable CTE of the carbon-doped HEAs can be ascribed to the relatively low CTE values of the formed B2, L1 0 , M 23 C 6 , and σ phases. The CTE value of a single face-centered cubic (FCC)-structured FeCoCrNiMn-1.3 at% C HEA is 16.7 × 10 –6  °C −1 , yet it can be continuously adjusted to 11.3 × 10 –6  °C −1 when the FCC matrix is gradually decomposed into the B2, L1 0 , M 23 C 6 , and σ phases. Furthermore, the decomposition rate and fractions of the B2, L1 0 , M 23 C 6 , and σ phases can be controlled via changing carbon concentration and rolling reduction. More importantly, the CTE range of the 1.3C HEAs meets the requirements of electronic packaging. This work provides a way to design HEAs with desirable CTE for electronic industry via annealing at intermediate temperature.