A method of operating a memory device includes applying an initial read voltage to a selected wordline to perform a read operation on memory cells connected to the selected wordline, determining whether a read failure occurs with respect to one or more of the memory cells, upon determining that a read failure has occurred with respect to some of the memory cells, determining threshold voltage distribution information for distinct groups of the memory cells, and determining a new read voltage to be applied to the selected wordline based on the threshold voltage distribution information.