The purpose of this research is the identification of issues related to constraints in the flexible manufacturing system (FMS). FMS comprises different resources as machine tool, material handling system with computer control, etc. FMS offers high utilisation of resources, less labour costs, less work-in-process inventories, shorter lead times, etc. Machine tools are required to produce different parts. Robots and automated guided vehicles (AGVs) are included in FMS material handling system. This paper shows the study of different constraints in FMS. Modelling of these constraints is performed by interpretive structural modelling (ISM), fuzzy ISM and total ISM (TISM) approaches. The driving and the dependence power of the constraints are analysed by MICMAC and fuzzy MICMAC. To validate the driving power of variables, the relative importance of variables is determined by AHP methodology. Finally, concluded that fixture and robot are the key variables of FMS constraints.