Hexagonal phase beta-NaYF4 thin films doped with Yb3+ and Er3+ have been made by electrodepositing onto beta-NaGdF4 underlayer usingY-EDTA, Yb-EDTA, Er-EDTAand NaF as precursors in an aqueous solution at lowtemperature. The underlayer substrates have large effects on their crystal phase. The hexagonal phase beta-NaYF4:Yb/Er can even be obtained at 30 degrees C if the films are electrodeposited onto electrodeposited hexagonal phase beta-NaGdF4 underlayer. Cubic-phase (a-phase) NaYF4:Yb/Er films are formed only if the films are electrodeposited onto tin doped indium oxide (ITO) glass. The formation of hexagonal phase beta-NaYF4:Yb/Er might be due to the epitaxial stabilization effect since beta-NaYF4: Yb/Er and beta-NaGdF4 crystals have same crystal structure and very small lattice parameter difference (less than 1%). XRD, SEM and HRTEM were used to characterize the structures of electrodeposited films. The hexagonal phase beta-NaYF4: Yb/Er films exhibited much higher intense visible light emissions excited by a 980 nm laser than its cubic phase. (c) 2019 The Electrochemical Society.