The superconductivity in the YBaCuO system can be explained in terms of the superconducting percolation of 90 K orthorhombic microdomains. Kubo et al. [ 1 ] have studied the percolation behaviour of the 123 system and estimated the total critical oxygen occupancy Pc to be 0.75 for the orthorhombic I structure using a 150 × 180 lattice model. In this paper, we report our work on the percolative behaviour of the 123 system, using a Monte Carlo method. We have studied the effect on Pc of increasing the lattice dimension up to 500× 500. For Pc-< 0.60 no percolative behaviour was observed, suggesting the tetragonal phase. Few times percolation was observed for 0.60 <P-< 0.65 indicating the phase transformation from tetragonal to orthorhombic. For 0.65 -< P< 0.77 (or 0.230 _< ~< 0.35 ) weak percolative behaviour was observed suggesting the formation of orthorhombic II structure, which is in good agreement with the value observed by Cava et al. [2]. For 0.77_<P_< 1.0 strong percolation was exhibited indicating the formation of orthorhombic I phase. We have explained the weak link region observed for 0.60 < P-< 0.77. We estimated the total critical oxygen occupancy Pc = 0.766 for an orthorhombic I structure for the lattice.