The effects of Na + doping level on the thermal conductivities, absorption and emission spectra, and fluorescence lifetimes of Yb 3+ ,Na + :CaF 2 crystals were systematically studied. Sites structure, covalent force, and crystal field strength of Yb 3+ :CaF 2 crystals were markedly varied by codoping Na + as charge compensator. The 2.0at% Yb 3+ and 3.0at% Na + -codoped CaF 2 crystal was demonstrated to operate in diode-pumped passively mode-locking scheme. Transform-limited 1ps laser pulses were obtained, showing the crystal capable of producing ultra-short laser pulses.