The objective of this study is to investigate the differences of AE signals caused by the matrix cracking and delamination. To accomplish this objective, tensile tests were performed with graphite/epoxy cross-ply composite specimens to detect AE signals caused by the matrix cracking. A Mode I test was performed with a graphite/epoxy DCB specimen to detect AE signal caused by the delamination. In these tests, AE signals were processed by STFT(short-time Fourier transform) that is appropriate for some non-stationary transient problems. In the tensile test, AE signals were detected by the EFPI embedded in the specimen, attached on the surface of the specimen and a conventional PZT. AE signals of the embedded EFPI were in better agreement with signals of PZT than those of the attached EFPI. The EFPI was more sensitive when it was attached in 90˚ direction on the surface of the specimen than in 0° direction. In the case of matrix cracking, the characteristic frequency ranges were 20 ~ 60 kHz, 120 ~ 170 kHz, and 200 ~ 240 kHz. The amplitude of 20 ~ 60 kHz was the most dominant. In the Mode I test, AE signals were detected by the EFPI embedded in the DCB specimen and the PZT. These were in good agreement with signals of the PZT. In the case of delamination, the characteristic frequency ranges were 60 ~ 80 kHz, 140 ~ 160 kHz, and 190 ~ 230 kHz. It was confirmed that there are some differences of AE signals caused by the matrix cracking and delamination in the frequency range and occurring time.