Neutron powder diffraction was applied to determine the deuteration level of K(H1−xDx)2PO4 (DKDP) crystals via Rietveld refinement method. Micro-Raman spectroscopy was employed using the neutron results to investigate the relation between deuteration level and PO4 vibration peak of DKDP crystal. The relative variation [Δν1 = ν1(KDP)−ν1(DKDP)] of the PO4 vibration peak was linearly well dependent on the deuteration level. Attenuated Total Reflectance-Infrared (ATR-IR) spectroscopy was also used to study the total relative variation [β(DKDP)-β(KDP) + ν1(DKDP)−ν1(KDP)] of β(O-H/D) and ν1(PO4) absorption band with the deuteration level of DKDP crystals. The IR results illustrate that two linear relationships existed between the deuteration level and the relative variations of the spectra. The two spectroscopic techniques can be combined and used to measure the degree of deuteration in the crystals of DKDP grown from solutions with deuteration level of less than 92%. ATR-IR spectroscopy is more suitable for measuring highly deuterated DKDP crystals.