The development of solid-state electrolytes (SSEs) is crucial for realizing next-generation high-energy-density and high-safety lithium-ion batteries (LIBs). The chemical composition design and synthesis route are known to be the determining factors for SSE properties. One of the most promising SSEs for LIB applications is garnet-type Li+-conducting oxide synthesized via a solid-state reaction. This study performs an in situ TEM investigation of Li6.25Ga0.25La3Zr2O12 (Ga-LLZO) growth during a high-temperature calcination process. At 750 °C, an intermediate phase, La2Zr2O7 (LZO), is formed through epitaxial growth along the crystallographic orientations of (1‾11)LZO//(1‾11)ZrO2 and LZO//ZrO2. The incorporation of Li and Ga into LZO is found to occur at 900 °C. The LZO transforms into Ga-LLZO via a layer-by-layer diffusion process that takes place along the [011‾] direction. The Ga doping can stabilize the cubic structure of Ga-LLZO at a temperature of 900 °C (while a temperature of >1100 °C is needed to obtain cubic LLZO) and eliminate the formation of the unwanted tetragonal phase. This dynamic microstructure evolution of Ga-LLZO is examined at an atomic scale for the first time. This study opens up a new route to better characterize and understand SSE materials, providing opportunities for further tailoring SSE properties.