Image forming apparatus selectively applying eco mode

An operating method of an image forming apparatus includes: receiving an input to select a selective eco mode which selectively applies an eco mode during an image forming job of a document; obtaining at least one area to be excluded from application of the eco mode from a preview of the document according to execution of the selective eco mode; generating a mask in which an area corresponding to at least one area of the document is set in an array corresponding to a preview image of the document, based on information about the at least one area of the document; and outputting the document to which the selective eco mode is applied based on the mask.