The specimen under test is in the form of a rod with plane parallel ends. A thin annular groove is worked in the rod at about one‐third of its length. A transmitting‐receiving quartz crystal delivers ultrasonic pulses to the rod at its remote end. The pulse heights of the reflections from the constriction and the other end are measured on a cathode‐ray oscillograph. Assuming a plane‐wave propagation, a formula is derived to calculate the attenuation constant of the short portion of the rod in terms of the mentioned pulse heights. The obtained formula is independent of the length or attenuation of the long portion of the rod. This enables one to heat the short portion while the long portion is cooled, thus overcoming the difficulties of matching usually met with in high‐temperature measurements between a quartz transducer and specimen. The method is applied to measure the attenuation of longitudinal waves in pure aluminum at temperatures up to 700°K for frequencies of 3 and 5 Mc/sec, respectively. The results indicate a thermally activated internal friction whose value agrees with that previously obtained by Kâ at a frequency of 1 cps.