The Japan National Large Telescope (SUBARU) under construction adopted Corning ULE glass for its primary mirror, which is made from 44 hexagonal blank segments (hexes) sealed together. The optimized hexes placement pattern was determined using the simulated annealing method so as to minimize the thermal deformation due to the inhomogeneity of the coefficient of thermal expansion of the hexes. The sealing process has been successfully completed and now the flat-to-flat blank is under the grinding phase to be followed by the sag-down phase for meniscus forming. The present paper summarizes the hexes placement pattern optimization method and various analysis behind its actual application, including current status of the fabrication.