Abstract: The evaporation by decomposition of NH4NO3 microdroplet has been investigated under atmospheric pressure and temperature. During the experiments, a single microdroplet with an initial radius of 5-30 μm. was suspended in an electrodynamic balance and the change of its light-scattering intensity with time at a scattering angle of 34.5°-68.4° was measured by a 512 element linear photodiode array. The microdroplet size was determined by comparing the experimental data with Mie theory. The change of the microdroplet size in the process of evaporation by decomposition was thus determined precisely. A diffusion controlled surface reaction model based on the experimental data was proposed to describe this process. The process of evaporation by decomposition of NH4NO3 microdroplets can be described as follows: when a single microdroplet was suspended in an electrodynamic balance, moisture first evaporated with the decomposition of NH4NO3. As a result, a microdroplet of metastable pure NH4NO3 was obtained. Since it was thermodynamically unstable, the microdroplet would change into a solid crystal, after having experienced further evaporation.