The air core formed in the pressure-swirl atomizer has been experimentally and theoretically investigated. The experiments were implemented from the pressure of 0.1 MPa -1.4 MPa at the room temperature. Five types of atomizers with different swirl chamber length and five types of mixture of the glycerol and water with different liquid viscosity were employed for discussing effects of the swirl chamber length and the liquid viscosity on the air core size. In addition, a semi-theoretical prediction correlation of the air core size was derived by establishing a control-volume model based on the angular momentum theorem in the particles system, and then was verified with the experimental results. It is found that the air core size increases with the decrease of the swirl chamber length and the decrease of the liquid viscosity. The air core will disappear when the swirl chamber length is greater than a critical length or the ejected liquid viscosity exceeds a threshold if other parameters are unchanged. The critical swirl chamber length decreases with an increase of the liquid viscosity and the viscosity threshold decreases with an increase of the swirl chamber length. Compared with some available correlations proposed in the literatures, predictions by the new correlation derived in the present paper roughly show better agreement with the experimental results at a wide range of liquid viscosity. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.