We report a facile hydrothermal synthetic route to prepare a class of monodispersed lanthanide‐based compound submicrospheres with controllable size, which employs raw lanthanide oxides as starting material, urea as precipitator and poly(N‐vinyl‐2‐pyrrolidone) (PVP) as surfactant. Dependent on the intrinsic properties of respective lanthanide, the resulting products could be in the form of oxide, hydroxide or basic carbonate. These lanthanide hydroxides or basic carbonates can be easily transformed into their corresponding oxides by calcination, retaining the same morphology and size dispersion. The formation mechanism of these lanthanide‐based compound submicrospheres is investigated and PVP plays a critical role in forming uniform and well‐dispersed products. Furthermore, this method could be extended to a binary system by using two kinds of lanthanide oxides as starting material, resulting in doped‐type lanthanide oxide submicrospheres (such as Y2O3:Eu3+). The Y2O3:Eu3+ submicrospheres exhibit nearly uniform spherical morphology and narrow size distribution as well as good water solubility and sharp spectral emission at 610 nm (corresponding to the 5D0–7F2 transition of Eu3+). This makes them attractive materials for applications in fields such as fluorescent lamps, field emission displays (FEDs) or LCDs, or as biomedical labels and molecular probes.