A new π-bonded-chain-stacking-fault (π-SF) model is proposed for the Si(111)4x1-In surface structure. The model incorporates 4x1 Si(111) substrate reconstruction consisting of the sixfold Si rings in the faulted-unfaulted sequence connected through fivefold and sevenfold Si rings. Indium atoms (0.75 monolayer) reside above sixfold and fivefold Si rings, while sevenfold Si rings form π-bonded chains between In ridges.