Abstract: Postnatal developments of morphological features (forearm and body mass) and vocalization (call frequencies and durations) of Rhinolophus pusillus were studied in the Jishui Cave of Guilin City from May 31st to July 20th 2006. The morphological changes indicated that infants of R. pusillus grew significantly in the postnatal three weeks and then gradually slowed down. Both forearm length and body weight were significantly correlated with the growing of age. Durations of infants’ calls were fairly variable and longer than those of adults. No significant correlation was found between the duration and age. Spectral characteristics indicated that the calls produced by infants had rather variable spectrums, unstable frequencies, and multiple harmonics in the first 20 days after birth. Infants over three weeks old mostly emitted FM-CF-FM type calls. Frequencies of first harmonics were significantly correlated with their age but the second harmonics did not correlate with age. The call frequencies of the young five-week-old bats were similar with their adults. The call frequency had a significant linear correlation with forearm length. Our findings indicated that the high flexible vocal cord and undeveloped nasal chamber might affect the vocalization veracity of new born infants.