This work investigates negative bias temperature instability (NBTI) in low temperature polycrystalline silicon thin film transistors (LTPS TFTs) in a darkened and in an illuminated environment of different light intensities. Experimental results reveal that the generations of interface state density (N-it) are identical under various illuminated intensity NBTI stresses. Nevertheless, the degradation of the grain boundary trap (N-trap) under illumination was more significant than that for the darkened environment, with degradation increasing as illumination intensity increases. This phenomenon is mainly caused by the extra number of holes generated during the illuminated NBTI stress. The increased N-trap degradation leads to an increase in the darkened environment leakage current. This indicates that more traps are generated in the drain junction region than from carrier tunneling via the trap, resulting in leakage current. Conversely, an increase in N-trap degradation results in a decrease in the photoleakage current. This indicates that the number of recombination centers increases in the poly-Si bulk, affecting photosensitivity in LTPS TFTs.