In this work, the negative bias temperature instability (NBTI) in low temperature polycrystalline silicon thin-film transistors in a darkened and in an illuminated environment was investigated. Experimental results reveal that the generation of interface state density (N-it) showed no change between the different NBTI stresses. Nevertheless, the degradation of the grain boundary trap (N-trap) under illumination was more significant than that in the darkened environment. This phenomenon is mainly caused by the extra number of holes generated during the illuminated NBTI stress.