The Dual-Thrust Rocket Motors (DTRMs) are the most widely used solid propulsion system for anti-tank guided missiles. DTRM with intermediate nozzle is a special type of this system. Depending on the internal conditions, the flow through the intermediate nozzle can be sonic, subsonic or reversed. The objective of this paper is to understand the internal ballistics (IB) and phenomena of DTRM with sonic intermediate nozzle. In this study, a mathematical model describing the performance of this DTRM is derived from the main governing equations of internal ballistics. Also, a set of experiments is conducted using a developed test motor to inspect the physics of DTRM with sonic intermediate nozzle and assess the mathematical model validity. Results of the mathematical model have shown a good agreement compared with those from the experimental studies.