Abstract: In this paper, the performance degradation of FF fieldbus in noisy environment is analyzed through analyzing the effect of noises on token operation. Token mechanism is modeled by status machine, and detailed token operation under noisy environment is investigated through the stateus machine model. The mean token passing time and mean token rotation time are derived from the detailed token operation to evaluate the effect of noises on the performance of FF fieldbus. The analysis shows that the density of noise has a serious effect on the performance of FF fieldbus, and even tends to make FF fieldbus not meet the real time requirement any more.