The high peak-power of picosecond pulses produced by a self-mode-locked semiconductor disk laser can effectively improve the efficiency of nonlinear frequency conversion. This paper presents the intra-cavity frequency-tripling in a self-mode-locked semiconductor disk laser, and picosecond pulses train at 327 nm wavelength is achieved. The pulse repetition rate is 0.49 GHz and the pulse width is 5.0 ps. The obtained maximum ultraviolet output power under mode-locking is 30.5 mW, and the corresponding conversion efficiency is obviously larger than that of continuous-wave operation. This ultraviolet picosecond pulses have high spatial and temporal resolution, and can be applied in some emerging fields.