Photon upconverting luminescent hexagonal NaRF4:Tm (0.5 mol%) (R:Y3+,Yb3+) crystals with Yb3+ concentrations between 20 and 99.5 mol% were synthesized by a modified thermal coprecipitation method. The effects of the Yb3+ sensitizer concentration on the shape, size, structure, upconversion luminescence intensities and dynamic luminescence lifetimes were studied in detail. The intensity of ultraviolet upconversion luminescence at 340-365 nm upon 980 nm excitation increased up to 20 times with increasing Yb3+ concentration. The mechanisms for the changes in morphology, size and UV upconversion emission are discussed. In addition, the effect of the Tm3+ concentration on the UV emission of NaYbF4 crystals was investigated. The results demonstrate that the NaYbF4 with Tm3+ concentration between 0.4 to 0.8 mol% produce the most intense UV upconversion luminescence. These crystals may find applications in e.g. UV-visible solid state lasers or as an internal UV radiation source for many photochemical reactions.