A memory device and an operating method thereof adjust a slope of a word line voltage. The memory device includes a memory cell array including a plurality of cell strings, a voltage generating circuit configured to generate a word line voltage provided to a plurality of word lines, and a control logic configured to output a slope control signal adjusting a voltage level variation characteristic of the word line voltage provided from the voltage generating circuit, wherein, during a prepulse period of a read operation of the memory device, a slope of a first word line voltage provided to an edge group including one or more word lines, the edge group adjacent to a string selection line is greater than a slope of a second word line voltage provided to a center group including one or more word lines in a center region.