An ultrahigh-pressure furnace assembly using a multi-anvil-type apparatus, capable of conducting phase equilibrium studies up to 100 kbar and 1500 ¿C with temperature uncertainty similar to that of piston-cylinder apparatus, is reported. Temperature distribution within the furnace assembly was studied by a compositional mapping of diopsidic clinopyroxene coexisting with orthopyroxene and garnet. A graphite heater with a 5¿ tapered inner wall (3 mm O.D., 11 mm long) was found to give a temperature gradient less than 50 ¿C/mm, whereas that in a conventional straight-wall heater was greater than 200 ¿C/mm.