Abstract: 4-hydroxysemicarbazide (HSC, HL) could be used as the reducing agent for Pu(Ⅳ) in the Purex process. In this work, three complexes (Nd(HL)³⁺, \mathrmNd(\mathrmHL)_2^3+ and \mathrmNd(\mathrmHL)_3^3+ ) with the ratio of metal ion to HL at 1∶1, 1∶2 and 1∶3 were proved to exist through the coordination of HSC and Nd(Ⅲ) in aqueous solution by spectrophotometric titration method. Besides, the stability constants of the complexes were determined, as well as the coordination patterns of neutral molecules, which were verified by potentiometric titration. Single crystals, including the ligand, the perchlorate of the protonated ligand, and the 1∶3 complexes \mathrmNd(\mathrmHL)_3^3+ and \mathrmTb(\mathrmHL)_3^3+ were prepared by solvent evaporation method, respectively, with their structures determined by X-ray diffraction. In the above two complexes, the HSC is found to coordinate with the central metal by the carbonyl oxygen atoms and the oxime oxygen atoms, in a five-membered ring. Further, by comparing the length of the Ln-O (carbonyl) bond and the Ln-O (oxime) bond, it is proved that the proton on the oxime oxygen is not dissociated. By comparing the solved molar absorption spectra of the \mathrmNd(\mathrmHL)_3^3+ , the diffuse reflectance spectra of Nd(H₂O)₃(HL)₃·3ClO₄ crystals, and its absorption spectra in acetonitrile solution, combining the measurements of the fluorescence spectra and the fluorescence lifetime of Tb(H₂O)₃(HL)₃·3ClO₄ in the single crystal and in acetonitrile solution, it could be inferred that in the aqueous solution the proton on the oxime oxygen atom in HSC will not be dissociated, neither. Four novel crystals were synthesized in this study. Amongst, Nd(H₂O)₃(HL)₃·3ClO₄ was prepared by slow evaporation method in aqueous solution with the 1∶3 molar ration of Nd(ClO₄)₃ to HL, which is crystalized in R3c space group with unit cell: a=16.405 6(3) Å, b=16.405 6(3) Å, c=14.829 6(4) Å; α=90°, β=90°, γ=120°; Z=6. Likewise, the Tb(H₂O)₃(HL)₃·3ClO₄, which is also crystalized in R3c space group with unit cell: a=16.289 0(7) Å, b=16.289 0(7) Å, c=14.891 9(8) Å; α=90°, β=90°, γ=120°; Z=6. Besides, H₂L·ClO₄ was cultured in aqueous solution with the 1∶1 molar ration of HL to HClO₄, with Pna2₁ space group and the unit cell: a=8.718 6(8) Å, b=13.809 0(11) Å, c=5.051 9(4) Å; α=90°, β=90°, γ=90°; Z=4. HL was cultured in anhydrous ethanol, with P2₁/n space group and the unit cell: a=4.099 8(2) Å, b=10.154 0(4) Å, c=8.802 1(4) Å; α=90°, β=99.035(4)°, γ=90°; Z=4.