Crystal structure of Ca1.1Co1.9(PO4)2 was successfully determined from laboratory X-ray powder diffraction data (Co Kα) using direct methods and the Rietveld refinement. The crystal structure was found to be monoclinic (space group P21∕n, Z=4) with lattice dimensions of a=1.452 67(5) nm, b=0.494 34(1) nm, c=0.867 50(3) nm, β=92.316(1)°, and V=0.622 45(3) nm3. The final reliability indices calculated from the Rietveld refinement were Rwp=3.93%, Rp=3.02%, RB=4.10%, and S=1.48. Both Co and Ca atoms were distributed over the 2a and 2d sites with a preference of Ca at the 2d site. The coexistence of Co and Ca on the 2a and 2d sites is indispensable for stabilizing Ca1.1Co1.9(PO4)2 in the Ca3(PO4)2-Co3(PO4)2 system.