Well-ordered nitrogen-covered Si(001) surfaces were formed for the first time by thermal nitridation using an N-2/H-2 gas mixture. The scanning tunneling microscope images showed clear Si(001)-2 x 2 reconstruction over the entire surface. The ordered structure of the nitrided surfaces can be explained by a model in which the surface nitridation by N-2 and the termination of the remaining Si dangling bonds by hydrogen atoms occur simultaneously to minimize the surface free energy during the nitridation reaction.