Nonvolatile memory device includes memory cell region including a first metal pad and a second metal pad, peripheral circuit region including a third metal pad and a fourth metal pad, vertically connected to the memory cell region. The nonvolatile memory device includes a page buffer circuit including page buffers to sense data from selected memory cells, each including two sequential sensing operations to determine one data state, and each of the page buffers including a latch to sequentially store results of the two sequential sensing operations. The nonvolatile memory device includes control circuit in the peripheral circuit region, to control the page buffers to store result of the first read operation, reset the latches after completion of the first read operation, and control the page buffers to perform the second read operation based on a valley determined based on the result of the first read operation.