In a method of operating a ferroelectric device, a ferroelectric device including a first electrode layer, a ferroelectric layer and a second electrode layer that are sequentially disposed is provided. A first remanent polarization is written in the ferroelectric layer. An operating voltage is applied between the first and second electrode layers to write a second remanent polarization having a polarization value different from a polarization value of the first remanent polarization in the ferroelectric layer. An amplitude of the operating voltage varies within a voltage application time period and varies in a set voltage range.