The powdered injection system is a novel biomedical device for needle-free adminstration of DNA vaccines. One system, call the Venturi device, uses the venturi effect to entrain DNA-coated micron gold particles into an established quasi-steady supersonic helium jet flow and accelerate them into an appropriate momentum in order to penetrate the outer layer of the skin or mucosal tissue to achieve a biological effect. In this paper, computational fluid dynamics is utilized to simulate the complete operation of a prototype Venturi system. The key features of the gas dynamics and gas-particle interactions are presented. In particular, the mechanism for the particle entrainment is explored. The overall capability of the Venturi system to deliver the particles into modelled targets is discussed. The statistical analysis shows that a mean impact velocity of 695 m/s is achieved for representative gold particles (1.8 microm in diameter), with a penetration depth of 29.8 microm for epidermal DNA delivery.