Fin-slot propellant grains have been used in a variety of solid rocket propulsion systems. Advantages that can arise from using fin-slot propellant grains include a constant total burning surface area and thrust level, large burning surface area, large free volume, and greater reliability for ignition. In order to accurately predict the overall ignition transient for the reusable solid rocket motors (RSRM) of the Space Shuttle booster with head-end fin slots, it is necessary to have the knowledge of the flame-spreading rates in the fin-slot region. Therefore, this research was aimed at obtaining a better understanding of the flame spreading phenomena in a motor with fin slots. A subscale (1:10) pie-shaped fin-slot motor was designed to simulate the first segment of the RSRM, and to perform diagnostic measurements for studying the flame spreading behavior on the exposed propellant surface. The motor contained a single triangular fin section mounted in a horizontal, 2-D axisymmetric stainless steel chamber with observation windows. Dynamic similarity was considered in the igniter design so the impinging jet had a similar exit angle onto the propellant surface in the fin-slot section. Flame-spreading measurements were gathered using a high-speed digital camera and non-intrusive optical measurement methods through an array of 36 near-IR fast-response photodetectors installed perpendicular to representative regions of the propellant surface. Results showed that flame-spreading phenomena was highly non-uniform, starting in the downstream portion of the fin-slot region before traveling back toward the igniter end. A correlation was developed for the flame-spreading time interval showing that the flame-spreading interval was inversely proportional to pressurization rate, which depends strongly upon the strength of the igniter induced flow and local propellant grain geometry.