Tohoku Electric Power Co., Inc. Haramachi Thermal Power Station No.1 coal-fired, supercritical, variable pressure operation oncethrough boiler with vertical furnace waterwalls has achieved the following successful results. This paper reports on the design features and trial operation results of this boiler. (1) A highly reliable vertical furnace waterwall has been employed using rifled tubing. (2) The advanced combustion technology features NO_X exhaust of 150 ppm and 3% of unburnt carbon in fly ash achieved through the use of a low NO_X burner, an in-furnace NO_X reduction system and a very fine pulverize. (3) A high boiler efficiency of 91.81% is obtained through improved combustion and less slagging. The boiler achieves high flexible operation capable of exclusive coal firing at 15% load.