A nonvolatile memory device includes a plurality of memory blocks and a control logic circuit configured to perform a first page on-chip valley search (OVS) operation on memory cells connected to one wordline of a memory block selected in response to an address, among the plurality of memory blocks, in response to a first read command. The control logic circuit is further configured to change a read level of at least one state using detection information of the first page OVS operation, and to perform a second page read operation on the memory cells using the changed read level in response to a second read command.