The ytterbium aluminum garnet composition YbAG (62.5 mol.% Al 2 O 3 , 37.5 mol.% Yb 2 O 3 ) was prepared in the form of glass microspheres by flame synthesis. Precursor powder for flame synthesis with high homogeneity was prepared by modified sol–gel Pechini method. XRD pattern of prepared glass microspheres indicated predominantly amorphous nature of the sample. Detailed study of morphology of the microspheres by scanning electron microscopy revealed the presence of a small fraction of partially or fully crystallized microspheres. The high-temperature X-ray powder diffraction analysis (HT XRD) was carried out in the temperature interval 750–1450 °C: The temperature dependence of phase composition was determined. Crystallization of Yb 3 Al 5 O 12 —ytterbium aluminum garnet phase—was observed in the temperature range 900–1200 °C. The DSC analysis with heating rates 2, 4, 6, 8, 10 °C min −1 in temperature interval 25–1200 °C was performed in N 2 atmosphere to study thermal behavior and crystallization kinetics of prepared glass microspheres. The two exothermic effects at 918 and 939 °C were observed, which were attributed to Yb 3 Al 5 O 12 crystallization. The crystallization kinetics of prepared sample was examined with the use of JMAK model, and the kinetic triplet—frequency factor A  = (1.8 ± 2.2) 10 +28 min −1 (for the first peak), A  = (1.2 ± 1.6) 10 +55 min −1 (for the second peak), apparent activation energy E app  = (6.4 ± 0.1) 10 +02  kJ mol −1 (for the first peak), E app  = (1.3 ± 0.1) 10 +03 kJ mol −1 (for the second peak) and the Avrami coefficient m  = 3 (for the first peak) and m  = 2 (for the second peak)—was determined using RSS, R adj 2 , AIC and W AIC criteria.