An atmospheric pressure ammonia storage tank with capacity of 30,000 MT located at Taiwan Fertilizer Company (TFC) Kaohsiung Factory were installed in 1987. After 27 years of service (1987~2014), TFC decided to decommission and demolish this ammonia tank due to new development plan of converting Kaohsiung Factory site to a business district. As the domestic contractors do not have any experience of handling huge ammonia tank decommission project, the project planning, a system design and an implementation were carried out by TFC project team. The project has been executed successfully in mid of 2016 without causing any environmental impact or industrial safety issues. This paper described in detail about the planning process, decommission plan, execution of the job, internal inspection findings and the subsequent review along with recommendations. We hope this article can be a valuable reference to the chemical industry professionals when planning inspection or internal repair jobs on the huge chemicals storage tank decommission.