An operation method of a nonvolatile memory device which includes a memory block having wordlines includes performing an erase on the memory block, performing a block verification on the memory block by using a 0-th erase verification voltage, performing a delta verification on the memory block by using a first erase verification voltage different from the 0-th erase verification voltage when a result of the block verification indicates a pass, and outputting information about an erase result of the memory block based on the result of the block verification or a result of the delta verification. The delta verification includes generating delta counting values respectively corresponding to wordline groups by using the first erase verification voltage, generating a delta value based on the delta counting values, and comparing the delta value and a first reference value.