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The temporal behavior of the very dim optical afterglow of GRB 080503 is atodds with the regular forward shock afterglow model and a sole kilonovacomponent responsible for optical emission has been speculated in someliterature. Here we analyze the optical afterglow data available in archive andconstruct time-resolved spectra. The significant detection by Keck-I in {\itG/R} bands at $t\sim 3$ day, which has not been reported before, as well as thesimultaneous Gemini-North {\it r} band measurement, are in favor of a power-lawspectrum that is well consistent with the optical to X-ray spectrum measured at$t\sim 4.5$ day. However, for $t\leq 2$ day, the spectra are thermal-like and astraightforward interpretation is a kilonova emission from a neutron starmerger, making it, possibly, the first detection of a very early kilonovasignal at $t\sim 0.05$ day. A non-thermal nature of optical emission at latetimes ($t\sim 2$ day), anyhow, can not be ruled out because of the largeuncertainty of the {\it g}-band data. We also propose to classify the neutronstar merger induced optical transients, according to the temporal behaviors ofthe kilonova and the non-thermal afterglow emission, into four types. GRB080503 would then represent the first observation of a sub-group of neutronstar merger driven optical transients (i.e., Type IV) consisting of an earlyblue kilonova and an adjacent non-thermal afterglow radiation.

GRB 080503: A very early blue kilonova and an adjacent non-thermal radiation component