An operating method of a memory device is provided. Using a statistical model, a resistance Rdyn of a variable resistor of a memory cell and a variation ΔRdyn of the resistance Rdyn are determined. Based on the resistance Rdyn and the variation ΔRdyn of the resistance Rdyn, an average resistance Rdyn_avg and a beta value of the variable resistor are determined. Then, using the average resistance Rdyn_avg and the beta value, a resistance Ra of an insertion resistor, connected between the memory cell and a power supply generator for generating a power supply voltage VPGM, is determined.