A numerical discrete fracture flow approach was used to evaluate the effect of block size on the permeability of jointed rock.. The following conclusions were made: (a) Chances to reach REV size associated with the equivalent continuum behaviour for a jointed rock block increases with the block size . (b) REV size decreases with increasing joint density and joint size, (c) REV size does not exist for joint systems with low relative orientation angles and low densities, (d) The first invariant of fracture tensor (FO) can be used to estimate the joint geometry requirements for the REV size associated with the equivalent continuum behaviour. (e) Average block permeability seems to be related to the first invariant of fracture tensor and the joint geometry requirements for non zero block permeability can be determined from the threshold value of FO. (f) A strong correlation exists between the directional coefficient of permeability and the fracture tensor component for the connected joint configuration.