A package structure includes a package substrate, a first die, a second die, a first underfill, and a second underfill. The first die and a second die are disposed on the package substrate. The first underfill is between the first die and the package substrate, and the first underfill includes a first extension portion extending from a first sidewall of the first die toward the second die. The second underfill is between the second die and the package substrate, and the second underfill includes a second extension portion extending from a second sidewall of the second die toward the first die, the second extension portion overlapping the first extension portion on the package substrate.