"Vibrato" is a certain periodic variation of sound properties. It is classified into three kinds : frequency vibrato, amplitude vibrato, and spectrum vibrato. The relations between hearing and various factors of frequency vibrato were investigated and the fundamental frequency (f_0) of each sound stimulus used was 440 Hz in this investigation. Initially the principal pitch and pleasantness of the FM sounds were examined as a function of the rate of vibrato (p_0) and extent of vibrato (2×Δf) in the case of pure tone. From the results of the research on principal pitch, the relation m=F(p_0/Δf) was obtained (Fig. 4). This was the same as the result obtained by K. Hirose (Table 2). However, when Δfwas small, a different tendency was observed (Fig. 5). From result of the study on the pleasantness of pure tone with frequency vibrato, the average pleasantness was maximum at p_0=7 times/sec and Δf=0〜2 Hz. The smaller or larger than 7 times/sec p_0 or the wider Δf was, the lower the average pleasantness became (Fig. 8, Fig. 9) . In comparison with Seashore's research on frequency vibrato of the tones of music performances, our results were the same with respect to p_0 but different with respect to Δf. Seashore demonstrated that the maximum preference of frequency vibrato was observed when Δf was about a quarter-tone in singing voices, and one-eighth in musical instruments like the violin. In the following investigation, p_0 was fixed at 7 times/sec. The next study concerned the relation between Δf and the pleasantness of complex FM sound with 5 harmonics whose spectrum envelope shapes were flat. The average pleasantness was maximum when Δf was 0〜1 Hz, and the wider Δf was, the lower the average pleasantness became as in the case of pure tone (Fig. 9). When we changed the shapes of the spectrum envelope of the complex FM sounds with 5 harmonics, we found a tendency for the average pleasantness to become higher as the shape became similar to pure tone (Fig. 10). Finally, from the results of the study on phase inversions among the AM signal waves of the 5 harmonics of a complex sound, the differences of this phase inversion could be detected and the average pleasantness was maximum in the case of the sound stimulus whose phases of the AM signal waves of its harmonics were made inverse between odd and even harmonics (Fig. 11). A paired comparison test was used for the measurement of pleasantness. These results will not be directly applied to the tones of music performances, but they will be useful in the case of designing sound signals, etc. .