Abstract: Differential thresholds of loudness (△I/I) were determined on 5 subjects for different signal durations (t) by both the signal comparison method and the method of beat perception.△I/I obtained by the first method did not seem to change with the change of signal duration (△I/I=K), whereas △I/I obtained by the second method varied systematically: when t was shorter than some critical duration T, △I/I were inversely proportional to the square root of t, i.e. △I/I=K'(T/t)2/2; when t was longer than T, △I/I became constant, i.e. △I/I=K'. T was about 150 msec. The results were compared with the previous data dealing with the role of signal duration in pitch differentiation.