We present an extensive comparative analysis on the growth, the spectroscopic features, and the laser performances of room temperature Yb(5%):LiYF 4 (YLF) and Yb(5%):LiLuF 4 (LLF) crystals in the same conditions. The gain cross section, fluorescence lifetimes, laser thresholds and laser slope efficiencies are compared for the first time. It appears that Yb 3+ -doped LLF crystal might be more promising for diode-pumped high power laser operation. Intensity, a.u. ×10 3 30 25 20 15 10 5 0 45 40 35 30 25 20 15 10 5 0 Time, μs YLF LLF YLF: (2.2 ms) LLF: (2.6 ms)Room-temperature fluorescence decay curves of the grown crystals at 1025 nm