Abstract: Estimating the activity of faults is not only the basis to predict earthquake and to reduce geological disasters,but also is the core issue of urban engineering design. Based on fuzzy and grey characteristics in assessment of fault activity,a fuzzy-grey model of assessment is proposed in the frame work of fuzzy mathematical theory associated with grey system. This paper establishes the fuzzy integrate assessment model and decides a trapezoidal membership function using the fuzzy mathematics. The power of each factor is decided with the principle that the relative value of activity symbol is larger,the power is greater. This model is applied to the assessment of fault activity in Xian area. The result shows that the activity of LintongChangan fault is the strongest and that of YuxiaTieluzi fault is the weakest.