A Rock Engineering Systems (RES) approach was adopted for rock mass characterization to indicate unstable slopes along the main roads of Bandarban Sadar Thana, Bandarban district of southeastern Bangladesh. The research work was based on a thorough field investigation, which was followed by geomathematical analyses of collected data. Fieldwork was carried out along two roads in Bandarban Sadar Thana of Bandarban. The slopes are composed of rocks of the lithostratigraphic units--the Surma and Tipam Group. Data on probable parameters related to slope instability were collected for the analyses. Based on the findings of the field investigation, literature review and collected ancillary data, 13 parameters were found relevant to slope instability. For the geomathematical analysis, matrix coding was done and rating values were designated for subcategories of each parameter to construct pull down menus. Finally, Rock Mass Instability Index (RMIIj) was calculated for each slope; higher values of RMIIj indicate higher degrees of slope instability. For the purpose of verification of the RES methodology and results, several stable slopes were included in the field study and geomathematical analyses. It was found that the unstable slopes had a RMIIj value ranging between 43.99 and 65.27, whereas the stable slopes were found to have RMIIj values between 14.04 and 39.10. Therefore, the adapted RES method of rock mass characterization based on RMIIj values was found effective in differentiating the studied slopes based on their degree of slope instability.