High entropy alloy (HEA) is a new kind of metallic materials composed of five or more elements with relatively large proportions. A precipitation hardening high-entropy alloy (FeCoNi)₈₆Al₇Ti₇ was fabricated by laser powder bed fusion (LPBF), aging heat treatments were performed to induce precipitation of two strengthening phases, coherent L1₂ and incoherent L2₁ phase. Yield strength of the HEA increases substantially from 710 MPa (as-printed state) to 1203 MPa (aged state). Theoretical analyses reveal that the coherent L1₂ precipitate contributes most of the improved strength for the aged HEAs, whereas recovery during ageing causes the decrease of dislocation density thus exerts a softening effect. In addition, it is found that ductility decreases with increasing volume fraction of the incoherent L2₁ precipitates. Based on a void growth model, the trend is qualitatively explained. Moreover, a new fracture mode, intercellular fracture, is proposed to account for the strong dependence of fracture dimple size on the dislocation cells. The new facture mode is also directly confirmed by delicate microstructural examination. The findings provide an effective strengthening method and propose a unique fracture mechanism for the additively manufactured HEA.