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Generation of picosecond terahertz (THz)-wave via four-wave mixing in a photonic crys (PCF) was demonstrated.This was accomplished by using a dispersion-flattened octagonal photonic crystal fiber (O-PCF), which was reported with such major attractive features as low confinement loss, smaller effective area, and wideband single-mode operation.Perfect phase matching was achieved by operating near zero-dispersion wavelength (1.55 um) in O-PCF.Using this method, when the energy-conservation and phase-matching conditions were satisfied, THz waves can be generated across the frequency range 7.07-7.74 THz.As a result of that, both phase-matching curve and related THz frequencies are shown in Fig.1 (a).Through changing the length of O-PCF ranging from 5.30 cm to 7.30 cm, the peak power of 7.42 THz-wave has got a maximum value of 2.55 W at 6.25 cm.Consequently, the peak power of 2.55 W and the peak conversion efficiency of more than-66.65 dB at 7.42 THz are realized with a pump peak power of 2 kW in a 6.25-cm-long O-PCF.Moreover, the temporal domain pulse of the THz waves tuned from 7.07 THz to 7.74 THz along a 6.25-cm-long O-PCF is shown in Fig.1 (b).Although these tuning ranges of THz waves are narrow, we could use dispersion-controlled PCFs to obtain wide tuning characteristics.This method and results demonstrate a promising way to realize a reasonable, easy to use, compact THz-wave source.