Provided is a word line structure including a substrate, a stack structure, and a metal silicide structure. The stack structure is disposed on the substrate. The metal silicide structure is disposed on the stack structure. The metal silicide structure includes a first metal element, a second metal element, and a silicon element. The first metal element is different from the second metal element, and concentrations of the first metal element and the second metal element gradually decrease along a direction from a top surface of the metal silicide structure to the substrate.