ISO 12135, which describes the method for the fracture toughness test, requires that the specimen temperature be held at the test temperature for 30 s per 1 mm of specimen thickness before testing to ensure a constant temperature through the full specimen thickness. However, this requirement results in an inefficient practice when testing thick specimens; e.g., a specimen with a thickness of 100 mm must be held at the test temperature for 50 min. Considering the high thermal conductivity of metals, the authors considered this requirement is excessively conservative, and therefore developed a new decision method for the temperature control time based on a heat transfer analysis. In the proposed method, the temperature control time is decided based on the material properties and temperature condition as well as the specimen thickness, and the required time is much shorter than that required by the ISO standard for general purposes. Cooling tests were conducted to investigate the effectiveness of the proposed method. The temperature history was measured at the surfaces and mid-thickness, and as a result, the proposed method showed more rational required times than the ISO requirement. The proposed required time was overestimated in most tests, however, these errors are significantly smaller than that of conventional one. Although some results were underestimated by a few seconds, this is not a problem because the values were sufficiently small. Based on these results, we concluded that the proposed method is more appropriate than the requirement in the ISO standard.