Reservoir-type microcapsules were produced in a mild and highly efficient manner using a coaxial ultrasonic atomizer. To understand the microencapsulation mechanism, the atomizer was operated in different modes. The results suggested the reservoir-type microcapsules were formed by midair collision of the microdrops of the two component liquids consisting of a polymer solution and an aqueous solution. The encapsulation efficiency and the distribution of the microdrops captured in midair were dependent on the ratio of the flow rates of the two liquids, which suggested that the collision involved multiple microdrops. This method allowed lysozyme to be encapsulated without loss of functional integrity and to be released with near zero-order kinetics for over 50 days. The ultrasonic atomizer provided a new method of preparing reservoir-type microcapsules.