A method of erasing a memory device, the method of erasing the memory device including: performing, in a first erase period, a first erase operation on memory cells respectively connected to a plurality of word lines, wherein at least one of the memory cells, which is included in a memory block, is not erase-passed; determining, after the first erase period, an erase operation speed by applying a verify voltage to at least one of the plurality of word lines, and determining an effective erasing time for each word line based on the determined erase operation speed; and performing, in a second erase period, a second erase operation on the memory cells respectively connected to the plurality of word lines based on the determined effective erasing times.