Chemical and morphological changes of phenol formaldehyde-based photoresist after $O_2$ radiofrequency(RF) plasma treatment depending on exposure time and source power were investigated. It was found that etch rate of photoresist sharply increased after discharge turn on and reached a limit with increase in plasma exposure time. Contact angle measurements and X-ray photoelectron spectroscopy(XPS) analysis showed that the surface chemical structure become nearly constant after 15 sec of the treatment. Atomic force microprobe(AFM) measurements were shown that surface roughness was increased with plasma exposure time.