Hexagonal-phasecore-shell-structured NaYF4:Yb,Tm@beta-NaYF4:Yb,Er and beta-NaYF4:Yb,Tm@-NaYF4:Yb,Er@#NaYF4:Yb,Tm nanocrystals were synthesized by a seeded growth approach. beta-NaYF4:Yb,Tm nanocrystals with 20 nm diameter were used as seed crystals to induce the growth of beta-NaYF4:Yb,Er and then beta-NaYF4:Yb,Tm crystals, resulting in the formation of core-shell-structured nanocrystals with upconverting lanthanide ions Tm and Er doped in the core and shell, respectively.