Oil-dispersible α-NaYF4 spherical nanoparticles and β-NaYF4 hexagonal-shaped nanoplates were synthesized by the liquid−solid two-phase approach at different reaction temperatures. The TEM and FE-SEM images reveal that the nanoplates have a relatively narrow size distribution. In comparison with other methods, pure β-NaYF4 hexagonal-shaped nanoplates were prepared under a relatively mild condition. The nanoplates grew at the liquid−solid interface with slow crystallization rate, which may be preferable for achieving β-NaYF4.