We developed a facile approach for the fast synthesis of ultrasmall and monodisperse beta-NaYF4:Yb3+, Er3+ nanocrystals down to 5.3 nm with the assistant of oleate ligands, based on which the highly bright sub-10 nm beta-NaYF4:Yb3+, Er3+@NaYF4 core-shell nanocrystals with three order of magnitude increased upconversion emission were successfully obtained by a seed-mediated growth method. The reaction time and temperature dependent experimental results implied that the introduciton of extra oleate ligands can both promote the alpha-to-beta phase transition and impeded the crystal growth of NaYF4, which is due to the oleate ligand induced orderly arrangement of Y3+ and capping effect, respectively. It is expected that the oleate ligands mediated phase transition and size evaluation, demonstrated here in NaYF4:Yb3+, Er3+ NCs, is also applicable to other lanthanide-doped upconversion nanocrystals.