A semiconductor structure includes an isolation structure, a gate stack, a spacer and a patterned resist protective oxide. The isolation structure is formed in a semiconductor substrate, and electrically isolates device regions of the semiconductor substrate. The gate stack is located on the isolation structure. The spacer is formed along a sidewall of the gate stack on the isolation structure. The patterned resist protective oxide is located on the isolation structure and covers a sidewall of the spacer such that the spacer is interposed between the patterned resist protective oxide and the gate stack.