We report the antimony (Sb) doping effect in a prototype system of iron-based superconductors LaFeAsO1−y F y (y=0, 0.1, 0.15). X-ray powder diffraction indicates that the lattice parameters increase with Sb content within the doping limit. Rietveld structural refinements show that, with the partial substitution of Sb for As, the thickness of the Fe2As2 layers increases significantly, whereas that of the La2O2 layers shrinks simultaneously. So a negative chemical pressure is indeed “applied” to the superconducting-active Fe2As2 layers, in contrast to the effect of positive chemical pressure by the phosphorus doping. Electrical resistance and magnetic susceptibility measurements indicate that, while the Sb doping hardly influences the SDW anomaly in LaFeAsO, it recovers SDW order for the optimally-doped sample of y=0.1. In the meantime, the superconducting transition temperature can be raised up to 30 K in LaFeAs1−x Sb x O1−y F y with x=0.1 and y=0.15. The Sb doping effects are discussed in term of both J 1–J 2 model and Fermi Surface (FS) nesting scenario.