The luminance-time behavior of organic light emitting device (OLED) was measured using a photodiode and the low frequency noise-time spectrum from photodiode current is also measured and investigated. Square pulses noises were observed in time domain and the laws of their occurrences were obtained and studied. The square pulses are believed to be related to sudden change in carrier numbers in the device as part of degradation process. It shows that two dominant mechanisms take turn to generate the optical noise levels. The degradation process is postulated to describe the corresponding noise change. This optical noise observation is a direct reflection of polymer intrinsic degradation. (C) 2007 Elsevier Ltd. All rights reserved.