Abu Dhabi Oil Refining Company, Takreer has recently commissioned the Ruwais Refinery Expansion Project. The Grass Root refinery having capacity to process 417,000 BPSD (Barrels per Stream Day) of Murban crude. Commissioning & Startup is one of the most critical milestones and phase of projects in refinery. It is the last visible step of a project execution process. It moves the project from the "end of construction" to the "commercial operation" status. This paper focus the challenges associated with new refinery commissioning and start-up situations, an understanding of the tools and techniques that can be used to facilitate the effective planning, execution, control and management of these situations. The work process on a Refinery project involves basic engineering, detail engineering, procurement, construction & erection, precommissioning and startup. While each of these activities has its own importance but construction and adoption of successful precommissioning Strategy are the two crucial activities that determine a safe flawless startup. This paper address some the critical commissioing issue encountered while strating a grass root refinery and petrochemical complex.