A memory device having a resistance change material and an operating method of the memory device are provided. A memory device includes a memory cell array including first and second resistive memory cells, which store different data according to the change of their resistance; a buffer including first and second storage regions corresponding to the first and second resistive memory cells, respectively; and a control circuit receiving program data to be programmed to the memory cell array, comparing first data stored in the first storage region and second data stored in the first resistive memory cell, and as a result of the comparison determining one of the first and second storage regions as a storage region to which to write the program data.