High quality Er 3+ /Nd 3+ :LiYF 4 single crystals were grown by a Bridgman method. Their spectroscopic properties were studied to understand the Nd 3+ concentration effect upon excitation of an 800 nm laser diode. The intensest 2.7 μm emission was observed in the LiYF 4 crystal codoped with 0.99 mol% Er 3+ and 0.62 mol% Nd 3+ . Meanwhile, the emission intensity for the green up-conversion and 1.5 μm downconversion of Er 3+ decreased with increasing of the Nd 3+ concentration. The modified Inokuti–Hirayama model was used to analyze the decay curves of the 1.06 (Nd 3+ ) and 1.5 (Er 3+ ) μm emissions. The results indicated that the energy transfer process (Er 3+ : 4 I 13/2 + Nd 3+ : 4 I 9/2 → Er 3+ : 4 I 15/2 + Nd 3+ : 4 I 15/2 ) is mainly due to the electric dipole–dipole interaction. The energy transfer efficiencies between Nd 3+ and Er 3+ ions were calculated. All results suggested that the Er 3+ /Nd 3+ :LiYF 4 single crystals may have potential applications in mid-infrared lasers.