The Langmuir monolayer is widely regarded as a model for investigating biomineralization on biological membranes, but so far the crystallization process has been elusive. In this work, we study the crystallization process of CaCO3 on a stearic acid monolayer at the air-water interface by in situ Brewster angle microscopy and ex situ electron microscopy. It is shown that the final CaCO3 crystals are transformed from a particle precursor rather than directly from the solvated ions under the compressed stearic acid monolayer. SEM and TEM images reveal that the precursors are uniform spherical particles of amorphous calcium carbonate (ACC) with sizes smaller than 100 nm. The amorphous particles are produced in an early stage of mineralization and can be stabilized for at least 0.5 h. Subsequently, the ACC particles tend to aggregate with increasing population and are eventually transformed into the crystalline calcite phase with the elapse of time. This work is the first to clarify the crystallization process of calcium carbonate under the monolayer at the air-water interface by a direct observation of the ACC and therefore establish the roles of stearic acid in the process.