The effect of alumina content and heat treatment time, on microstructure and cooperative emission of oxyfluoride glass-ceramics were investigated. Two values (1.8 and 2.18) were selected for SiO2/Al2O3 ratio. The results indicated that alumina content had a significant effect on the phase separation and the viscosity of the glasses. Although crystallization happened in the glass with SiO2/Al2O3 ratio of 2.18 via spinodal decomposition phase separation, the other glass was crystallized in a homogeneous glass phase. Accordingly, the size and volume concentration of nano CaF2 crystalline phase which precipitates during the heat treatment depends on the glass's alumina content. Furthermore, the crystal size was raised dramatically with duration of heat treatment time in the high-containing aluminum oxide. In the other glass specimen, i.e., in the glass with a SiO2/Al2O3 ratio of 2.18, only a slight increase in the crystal size was observed by raising the heat treatment time. Increasing the time of heat treatment led to the incorporation of Yb3+ ions into CaF2 crystalline phase. Moreover, cooperative emission coming from a pair of excited Yb3+ ions was obtained and increased significantly with the heat treatment time. The results showed that the optimum heat treatment time was 48 h.