Chemistry of detrital chromian spinels provides us useful information on the petrological characteristics of their source ultramafic and related rocks. This method is applied to the Ilocos Norte ophiolite, north of Laoag City, northern Luzon, Philippines, of which petrological characteristics have not been well known. Cr# (=Cr/[Cr+Al] atomic ratio) of the detrital spinels from one of the rivers running through the Ilocos Norte ophiolite ranges from 0.4 to 0.8, possibly indicating moderately to highly depleted harzburgites for their source rocks. This further indicates a supra-subduction zone origin for the Ilocos Norte ophiolite. The spinel chemistry demonstrates that the Ilocos Norte peridotites are different from the Coto and Acoje peridotites of the Zambales ophiolite, western Luzon.