β-NaYF4:Yb,Er upconversion nanocrystals are synthesized by a hot-injection method employing purified α-NaYF4:Yb,Er particles with a size of approximately 2–3 nm as single-source precursor. The method allows to prepare either spherical nanoparticles with different sizes or nanorods by simply controlling the addition rate of the solution of α-phase particles and performing the synthesis either in the absence or in the presence of seed particles. The luminescence properties of the different particles are compared also including core-shell particles.