A program method, applied in a flash memory, includes the following steps. Firstly, a first memory sector and a second memory sector are selected, wherein the first and the second memory sectors respectively correspond to a first word line and a second word line. Next, a first operation phase and a second operation phase are determined. Then, the first word line is biased with a first setup voltage, and the second word line is driven in one of a program operation and a program-verification operation, in the first operation phase. After that, the second word line is biased with a second setup voltage, and the first word line is driven in the other one of the program operation and the program-verification operation in the second operation phase.