We experimentally demonstrate an ultra-sensitive and polymer-based two-dimensional photonic crystal (2D-PhC) cavity optical sensor. The device was fabricated with polymer on sil- icon substrate by using electron beam lithography (EBL) and operated near its resonance at 532 nm. Using 2D-PhC cavity optical sensor, reflection intensity was 60 times higher than with- out cavity. In addition, coating the sensor surface using layer-by-layer (LbL) assembly method- ology by deposition of cationic (poly(sodium 4-styrenesulfonate)) and anionic (poly(allylamine hydrochloride)) layers for produces a different reflection intensity change. By deposition of LbL layers using 2D-PhC cavity optical sensor, reflection intensity was decreased systematically and high sensitively (at least 1 layer (thickness: 2 nm [1])). Furthermore, using this optical sensor, high sensitive detection of DNA hybridization could be achieved.