A memory having a memory cell, a resistance estimator and a write current generator. The resistance estimator is coupled to the memory cell to estimate the resistance of the memory cell and outputs an estimated resistance level. According to the estimated resistance level, the write current generator generates a write current to flow through the memory cell and to change the resistance of the memory cell. The write current is in a pulse form, and the write current generator sets the pulse width, or magnitude, or both the pulse width and the magnitude of the write current according to the estimated resistance level.