The present invention provides a label-free sensing chip for identifying a chemical substance, comprising: (a) a transparent substrate comprising a base and first periodic ridges; and (b) a metal layer covering said transparent substrate, comprising second periodic ridges and third periodic ridges, in which said second periodic ridges has a height equal to or greater than the height of the first periodic ridges, and each ridge of the second periodic ridges fits into the space between each ridge of the first periodic ridges, and said third periodic ridges correspondingly located on said first periodic ridges. The present invention also provides a method for identifying a chemical substance by using the foresaid label-free sensing chip.