The erosive burning effects in a small test motor loaded with practical composite propellants have been investigated in detail by using an X-ray absorption diagnostics to measure the propellant local regression. The motor was specially designed to have two propellant slabs and was called as DSM (Double Slab Motor). Significant erosive burning was forced to occur in the motor by varying combustion pressure and mass flux in the port. A new simple correlative equation is derived, which predicts accurately not only pressure history but also local regression of burning surface. The equation is characteristic in the non-dimensionalized threshold mass flux and includes the terms to consider both effects of pressure and of motor scale.