Hollow capsules have been intensively investigated due to their high capacity of encapsulating large quantities of guest molecules, making them promising candidate materials for various encapsulation applications. In this work, CaCO3 hollow capsules were successfully synthesized via an emulsion route. The interior hollow structure of the capsules was confirmed by using scanning electron microscopy and transmission electron microscopy (TEM). The vaterite polymorph of the as-synthesized CaCO3 capsules was determined by using x-ray diffraction, high-resolution TEM, and Fourier transform infrared spectroscopy. A self-assembly model was proposed to explain the formation mechanism of the vaterite capsules. By adjusting experimental parameters such as the internal solution amount and the surfactant amount of the double-emulsion system, the average capsule size could be adjusted accordingly. However, the increase in capsule size was at a compensation of size-uniformity degradation. The capsule size uniformity was then further optimized by increasing the magnetic stirring rate. The resultant vaterite capsules demonstrated biodegradability behavior after immersion in phosphate-buffered saline solution, leading to their promising applications in the area of controlled drug delivery.