In a method of operating a nonvolatile memory device including a memory cell array, where the memory cell array includes a plurality of pages, and each of the plurality of pages includes a plurality of nonvolatile memory cells, a first sampling read operation is performed to count a first number of memory cells in a first region of a first page selected from the plurality of pages, using a first default read voltage and a first offset read voltage, and a second sampling read operation is selectively performed to count a second number of memory cells in a second region of the first page, using the first default read voltage and a second offset read voltage, based on a comparison result of the first number and a first reference value. The second offset read voltage is different from the first offset read voltage.