Abstract: in order to characterize the connectvity of the milimetr sized cracks in the coal and obtain thecontribution of the permeabiiy to whole calspermeabilitywith obsevation and satstic of a o ofcal samples under microsope,the Monte Carlo method was appiedto the reconstruction of the millimeter sized crack network. n combinaton withthe Matab software,the effctive conetive crack network coud be obtained withthe zero given metod. The efifctive permeatiliy of the milmneter sized crack network was obtained with 2D seepage equation.i n comparison with the sie measured permeaiiltythe results showed tat the efective conectiveiimeter sized crack networ in the coal and rock would be a major contrbutor of permeabilt , Under the conditon tat the other parameters were constant with the density and with of the iimever sized cracks increased,the permeability would be increased in an index 'ype . When the length of the milimeter szed cracks inreasedto a cerain degre. permeability would no increase . he new cracks and increased crack length ocured in the reservoireonstruction would be the key to increase the fow capacity.The reconstucted iimeter szed crack network could clearly shoiwl the connectivity of the cracks and wauld set up the base to select the migration access of the coalbed methane crack network and to study the flow mode.