The behavior of bubble formation from a single orifice in a nonaqueous liquid and a liquid-solid suspension with pressure fluctuations in the gas chamber (plenum region) is studied at high pressures (up to 8.3 MPa). an analytical model is developed to account for the initial bubble size in liquid-solid suspensions at high pressures. The model takes into consideration various forces induced by particles, such as suspension inertial force. Both experiments and model predictions indicate that the initial bubble size in the suspension is generally larger than that in the liquid, and that it increases with an increase in the solids concentration. The system pressure has a significant effect on the bubble size in liquids or liquid-solid suspensions when bubbles are formed under variable gas flow-rate conditions, and a negligible effect under constant gas flow-rate conditions. This model can reasonable describe the initial bubble sizes under high-pressure conditions measured experimentally in this study and those reported in the literature.