The paper relates to the application of some advanced operation research techniques to probabilistic risk assessment (PRA) studies of selected plants in fertilizer complex. Hazard prone sections of ammonia plant are identified using Fire & Explosion Index, Toxicity Index and Mend Index. Past accident data analysis with emphasis on failure mode and critical evaluation of Urea plant accident have been discussed. Though probability of occurrence of hazardous event for vulnerable units is studied through fault trees analysis, the uncertainty with respect to top event in fault tree analysis is mitigated with recourse to Fuzzy sets theory(1). The process hazard for the total system is assessed using failure logic diagrams which simplifies the process by determining minimum cut sets. The reliability of various units is studied as a function of time through construction of pathway reliability block diagram. In order to maintain the desired minimum reliability the procedure for estimation for each path, preventive maintenance scheduling is explained.