Hexagonal NaYF4 crystals co-doped with different concentration of Yb3+ and Er3+ ions were synthesized via hydrothermal reaction route. The samples doped with 30mol% of Yb3+ and 1mol% of Er3+ showed the highest upconversion efficiencies under 980nm excitation. Powder X-ray diffraction (XRD) analyses indicated that the samples were pure hexagonal phase. The morphology of the powder was observed by scanning electron microscopy (SEM). With the excitation of a 980nm diode laser, strong green emissions were obtained at 522, 530, 542 and 551nm in addition to weak red emission at 654nm. The green and red emissions were attributed to the transitions of 4S3/2, 2H11/2 → 4I15/2 and 4F9/2 → 4I15/2 of Er3+ ion, respectively. The additions of Yb3+ ion as sensitizer can notably enhance the upconversion efficiencies from infra-red to visible. The upconversion mechanism of the emission for the samples was also discussed.