A package structure includes a first package component including an integrated circuit (IC) die, a substrate, a redistribution structure, and an antenna structure electrically coupled to the IC die through the redistribution structure. The substrate includes a semiconductor material and wider than the IC die, and the redistribution structure is disposed over a first side of the substrate and electrically coupled to the IC die. The package structure further includes an underfill interfaced with a second side of the substrate opposite to the first side, and the underfill covers a sidewall of the first package component.