Long-time average spectrum (LTAS) was used to analyze the cry phonations of 26 infants under four months old; 16 of them are full-term and the other 10 infants are preterm. The results of first spectral peak, mean spectral energy, spectral tilt, high frequency energy were used to compare the cry phonatory between full-term and preterm infants. In addition, cry duration and percent phonation is also compared. According to previous studies, full-term and preterm infants' crying behavior show significant differences because immature neurological development of preterm infants. Major findings in this study are: (1) There was no significant difference in unedited cry phonation across groups; (2) There was no significant difference in percent phonation across groups; (3) There was no significant difference in first spectral peak across groups, and no significant difference within groups could be found. However, full-term infants have higher first spectral peak than that of preterm infants; (4) There was no significant difference in mean spectral energy across groups, yet there was a significant main effect for partition; (5) There was no significant difference in spectral tilt across groups. Post hoc comparisons identified higher spectral tilt in P2 than in P3 in the full-term infants; (6) There was no significant difference in high frequency energy across groups. Significant differences were observed across partition, and in both groups, P1 had higher HFE than P3. The differences in the measures of crying behavior between full-term and preterm infants can help to estimate health condition of infants who are under 4 months old.