A continuous synthesis method for the less than 10 nm core–shell upconversion nanoparticles was developed via coiled tube embedded flask reactors and a flow solvothermal co-participation reaction up to 300 ℃. Fast nucleation of hexagonal nanocrystals in less than 9 min residence time was achieved owing to the excellent heating ability of the reactors, and a two-step reaction strategy was created for the synthesis of β-NaYF₄: Gd, Yb, Er/Ho/Tm@NaYF₄ particles without intermediate purification.