Permeability of the basalt and its scale effect have been discussed through three kinds of field hydraulic tests at the dam site of Xiluodu Hydroelectric Station. The permeability value K from water seepage test in adits with the test scale 1-2 m is dispersed between 10-4 and 101m/d. K from groundwater tracer test with the test scale 70-145 m is concentrated between 10-0.5 and 100.5m/d. K from water pressure test in borehole with the test scale 4-7 m is between 10-2 amd 100 m/d. Three kinds of test results indicate that K increases with the hydraulic test scale. This paper attributes the scale effect to heterogeneity of fractured rock. Small-scale tests are often performed in local matrix or single fracture with bad connectivity. Large-scale tests often cross a few large fractures or faults with better connectivity so that K is larger than the value from small-scale tests. When we study groundwater flow of fractured rock

for study areas with different scales

field hydraulic tests with corresponding scale should be selected to study the permeability of fractured rock.