1. A verifying method of an optical proximity correction (OPC), including: extracting, by a netlist processing device, a first netlist file from an integrated pre-OPC layout; merging, by a layout processing device, a first post-OPC layout and a second post-OPC layout to be an integrated post-OPC layout; extracting, by the netlist processing device, a second netlist file from the integrated post-OPC layout; and comparing, by the netlist processing device, the first netlist file and the second netlist file.