The radiometric temperature measurement is often applied to the in-situ and real-time monitor for rapid thermal processing of semiconductor wafer. To obtain good accuracy, the effective emissivity of measured spot is determined simultaneously as well. However, the effective emissivity strongly depends on the characteristics of wafer, processing chamber, and sensors. This paper presents a Monte Carlo model with bi-directional reflection distribution function to estimate the related effective emissivity of wafer. The ends of radiation thermometer considered are located either on the inner surface of processing chamber or at the proximity of wafer. The results are checked and compared with those of the previous work. Finally the primary effects on radiometric temperature measurement are analyzed and discussed.