The term gas-to-liquids refers to a small number of technologies designed to convert natural gas to liquid fuels, as alternatives to the traditional refining of crude oil and other natural gas commercialization routes. This work is a guideline on the theory of Process plant of GTL, i.e. the theory of synthesis gas production, Fischer-Tropsch synthesis and upgrading/refining. The plant is composed of process and mechanical equipment. Additionally, I considered the process plant design (i.e. design of the process and mechanical equipment’s) safety and environment, technical challenges associated with the process plant of gas to liquid (GTL). The Syngas production unit, Fischer Tropsch synthesis, refining of the GTL liquids to transport fuels (Product up-grade) have been modelled (simulated) using Aspen HYSYS software. The results of this simulation and the actual results obtained in experiment (Choi et al, Bechtel. 1996) were compared and are practically in reasonable agreement with each other. The book is especially useful to engineering students, Petroleum and gas engineers, Process and chemical engineers, and Mechanical engineers in the oil and gas sector.