Rock mass classification systems consider one of the design tools, which are used in conjunction with engineering assessments and other design approaches. There are many classification systems, which are widely employed in rock engineering. In this study one of these systems is used for the selection of the optimum panel width in phosphate mine Abu Tartur area. Geological Strength Index (GSI) is one of these systems which enables for calculations of the panel width. Data for the GSI system are obtained from geological reports, some field measurements and laboratory tests. The obtained panel width (wall length) for Abu-Tartur area is calculated to be about 100m (102m) which differs strongly from the applied length in the area (150m). So, it is recommended to apply this obtained length to secure safe mining conditions without roof falls which is the main problem facing underground mining in this area.