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High-content interstitial carbon addition has been proven to regulate the dislocation behavior in high-entropy alloys (HEAs) during deformation, thereby overcoming the strength-ductility trade-off, but its exact mechanism is still unclear. In this work, the dislocation generation at incipient plasticity in a high carbon-added Al10(FeNiCoMn)90 HEA was investigated by instrumented nanoindentation. It was found that interstitial carbon addition could increase the atomic cohesion, which induced a higher maximum shear stress and activation volume required to trigger the incipient plasticity related to dislocation nucleation. However, it decreased the activation energy for a critical dislocation loop during deformation, increasing the dislocation density and facilitating dislocations multiplication. Consequently, the plastic zone underneath the indenter expanded and pop-in events occurred in the carbon-added HEAs. These findings contribute to understanding how interstitial carbon regulates the dislocation behavior in HEAs.