A uranyl compound, K_4UO_2(CO_3)_3 has been characterized by powder X-ray diffractionmethod. M. W.=606.46, monoclinic, C2/c (No. 15), a=1.0240(7), b=0.9198(4), c=1.2222(12)nm,β=95.12(4)°,V=1.1466(5)nm~3, Z=4, D_m=3.468g/cm~3, D_c=3.513g/cm~, λ(Cu Kα_1)=O.1540598nm,T=298K. The structure was solved by heavy atom method and Fourier synthesis, and refined by full-matrix least-squares method to R=0.1185 for 275 reflections. The uranium (Ⅵ) atom is in aneight-coordinate distorted hexagonal-bipyramidal environment with creasy fan shape. The linearuranyl group approaches to perpendicular to the equatorial plane in which three carbonate groups arechelated. U(Ⅵ) has two linear oxygen atoms closer to it (U-O=0.1767 (5) nm) than six otherneighbours (U-O ranging from 0.2516 to 0.2568nm). The distances between carbon atoms anduncoordinated oxygen atoms are 0.122 (1) and 0.123(1) nm, which are distinctly different from thosebetween carbon and coordinated oxygen atoms (mean 0.134(6) nm). This fact reveals the non-eq-uivalence of one oxygen atom to the other two in each carbonate. In K_4UO_2(CO_3)_3, the O-O dis-tance for the adjacent carbonate groups is 0.2794(4)nm approaching to the sum of Van der Waals radiiof two oxygen atoms. The K-O distances vary between 0.2667 and 0.3131nm, and each anion isimmediately surrounded by six potassium ions, only four of which can be considered to belong to thesame structural formula unit, and they are symmetrically located above and below the equatorialplane. A uranyl compound, K_4UO_2 (CO_3) _3 has been characterized by powder X-ray diffraction. MW = 606.46, monoclinic, C2 / c (No. 15), a = 1.0240 (7), b = 0.9198 1.2222 (12) nm, β = 95.12 (4) °, V = 1.1466 (5) nm~3, Z = 4, D_m = 3.468g / cm~3 and D_c = 3.513g / = O. 1540598 nm, T = 298 K. The structure was solved by heavy atom method and Fourier synthesis, and refined by full-matrix least-squares method to R = 0.1185 for 275 reflections. The uranium (VI) atom is in aneight- coordinate distorted hexagonal-bipyramidal environment with creasy fan shape. The linearuranyl group approaches to perpendicular to the equatorial plane in which three carbonate groups arechelated. U (Ⅵ) has two linear oxygen atoms closer to it (UO = 0.1767 (5) nm) than six The distances between carbon atoms and uncoordinated oxygen atoms are 0.122 (1) and 0.123 (1) nm, which are distinctly different from those between carbon and coordinated oxygen atoms (mean 0.134 (6) nm) This In K_4UO_2 (CO_3) _3, the OO dis-tance for the adjacent carbonate groups is 0.2794 (4) nm approaching to the sum of Van der Waals radiiof two oxygen atoms. The KO distances vary between 0.2667 and 0.3131 nm, and each anion is temporarily surrounded by six potassium ions, only four of which can be considered to belong to the same structural formula unit, and they are located symmetrically located above and below the equatorialplane.