This study focuses on the understanding and modeling of the physical phenomena occurring in the degraded zones of Silica Phenolic (hereafter referred as SiFRP) under exposure to high temperature gases when applied to liquid rocket engine (LRE) combustor. Although the understanding and modeling of the phenomena is supposed to be essential in designing LRE combustor, a few works done in this fields appear in the open literatures. Basically, it is well known that when heated, the pyrolysis reaction proceeds in the SiFRP, forming 3 distinct zones of charred, decomposed and virgin zone, respectively. The obtainable information for the thermal response of SiFRP at ground firing tests is classified in 2 categories. The first is the equilibrium state characteristics after long time elapsed from the burnout, namely, the degraded thickness distribution, which reflects 3-D information (combustor inner surface×thickness-direction) of heat load distribution over the entire combustor inner surface thanks to the highly insulating nature of SiFRP. The second is the transient characteristics with regard to the degraded zones propagation in the SiFRP, which can be detected by application of ultrasonic testing (UT) method. In this paper, the progress of in-depth phenomena of SiFRP and the physical variation were intentionally studied. We strive to clarify and specify the quantitative threshold values of the interface points which characterizes each degraded zone and UT reflection point and eventually express the threshold values in terms of physical quantities that could appear in numerical analysis.