The heat generation values for the typical smoldering fire sources which have been proposed as the ISO Standard on Fire Detector Test were estimated experimentally based on oxygen consumption and weight loss rates. Heat generation values of smoldering wood and cotton wick showed 1/10 - 1/3 of those of flaming phase. The heat generation rate which was proposed as the Standard of ISO Fire Source was 1.5 kW and gave excess temperature of 2 - 2.5°C under the ceiling of the proposed ISO test room. It was found experimentally using the test room and proposed smoldering fire sources that the hot layer under the ceiling of which excess temperature of 2 - 3°C stopped the penetration of updraft smoke from the smoldering source through the thermal interface. It should be necessary that the exact definition on vertical thermal condition for the proposed ISO Test method to get repeatability, reliability, and sensitivity of the fire detector tests.