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To further improve the quality of the cuboid DKDP crystal, the effect of thermal annealing at 160 °C were evaluated. After thermal annealing, the symmetry of the HRXRD of the X-cut sample is much better, indicating that the uniformity of the cuboid crystal is improved. The transmittance of annealed THG-oriented sample is obviously higher than the as-grown THG-oriented sample from 400 to 300 nm. At 355 nm, the transmittance of the THG-oriented sample increases from 80.4% to 87.9% after thermal annealing. Besides, thermal annealing is also beneficial to the weak absorption properties at 355 nm of the X-cut cuboid DKDP crystal. The average weak absorption of the X-cut sample is decreased from 26.6 ppm to 12.3 ppm, and the standard deviation of the X-cut sample is decreased from 17.0 ppm to 4.8 ppm after thermal annealing. The LID performance of the THG-oriented sample is improved through thermal annealing, and the fluences corresponding to 50% probability of LID is increased from 13.4 to 14.8 J/cm2 (@3ns, 355 nm). Also, the density of the defect with low LID threshold decreases by two-thirds. The above results indicate that thermal annealing is an effective means to further improve the quality of the cuboid DKDP crystal. • Through thermal annealing, the symmetry of the HRXRD of the cuboid DKDP is much better. • At 355 nm, the transmittance of the cuboid DKDP increases from 80.4% to 87.9%. • The average weak absorption of the cuboid DKDP is decreased from 26.6 ppm to 12.3 ppm. • The fluences corresponding to 50% probability of LID of the cuboid DKDP is increased from 13.4 to 14.8 J/cm2 (@3ns, 355 nm). • Also, the density of the defect with low LID threshold decreases by two-thirds. [ABSTRACT FROM AUTHOR]