Aggressor memory cells connected to one or more aggressor wordlines are grouped into aggressor cell groups by performing a read operation with respect to the aggressor wordlines based on one or more grouping read voltages, where the aggressor wordlines are adjacent to a selected wordline corresponding to a read address among wordlines of a memory block. Selected memory cells connected to the selected wordline are grouped into a selected cell groups respectively corresponding to the aggressor cell groups. Group read conditions respectively corresponding to the selected cell groups are determined and group read operations are performed with respect to the plurality of selected cell groups based on the group read conditions. The read errors are reduced by grouping the selected memory cells into the selected cell groups according to the change of operation environments.