| Permeability of fractured rock mass is one of the importent content of hydrology. Many projects and studies have a good relation with fluid flow in fracture rock mass. This paper based on seepage theory of discrete fracture network and propses a general method which determines the existence of REV.The ISRM divides the fracture into five classes according to their trace length and seven classes according to their distances and there are thirty-five classes after such a classification and using the software simulates these fracture rock masses. Survey regions fixed hydraulic gradient 1 and fixed length-width ratio 2:1 are also rotated in 10°intervals to calculate the equivalent permeability. And then 36 equivalent permeabilities in different orientations will be got in each region in order to evaluate a the smallest size.And whether or not the 36 permeablities can be plotted as a permeability ellipse in polar coordinates and REV is discussed and determined.By the study of the fracture rock mass, the major results are as follows:(1) REV is exists in the very low tractility frcture rock mass,only when their fracture spacing is less than 200 mm; REV is exists in the low tractility and the medium tractility fracture rock mass,only when their fracture spacing is less than 600 mm; REV is exists in the very high tractility fracture rock mass,only when their fracture spacing is less than 4000 mm;REV is exists in the high tractility fracture rock mass,only when their fracture spacing is less than 6000 mm.(2) The fracture rock mass REV existence has a close relationship with fracture geometric parameters. REV will change when trace length,space,distance, dip, angle of dip etc..(3) The same tractility fracture rock mass, the spacing is smaller, the REV existence's possibility is bigger, and the REV size is smaller.(4) The same spacing fracture rock mass, the tractility is higher, the REV existence's possibility is bigger, and the REV size is smaller.
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