Well-dispersed spherical lanthanum oxycarbonate (La2O2CO3) and La2O3 particles with a flower-like morphology were synthesized via a simultaneous polymerization-precipitate reaction, metamorphic reconstruction, and mineralization under a hydrothermal condition as well as subsequent calcination. The La2O2CO3 and La2O3 microspheres obtained consist of about 20 or 40 nm thick nanosheets as petals, respectively. They have an open three-dimensional mesoporous and hollow structure and possess a high surface area, large pore volume, and marked thermal stability. These novel porous structural lanthanum-based materials could be used as catalyst supports for many reactions.