Regarding the previous earthquake damages in offshore structures, as vital structures in the oil and gas industries, it is important that their seismic design will be performed with very high reliability. Accepting the Nonlinear Time History Analyses (NLTHA) as the most reliable seismic analysis method, in this paper an offshore platform of jacket type with the height of 304 feet and with a deck of 96 feet by 94 feet, and weighing 290 million pounds has been studied. At first, some Push-Over Analyses have been performed to recognize the more critical members of the jacket, based on the range of their plastic deformations. Then NLTHA have been performed by using the 3-components accelerograms of 100 earthquakes, covering a wide range of frequency and normalized to three Peak Ground Acceleration levels of 0.3g, 0.65g, and 1.0g. By using the results of NLTHA the damage and probability of critical members have been studied to assess the reliability of the jacket structure. Regarding that different structural members of the jacket have different effects on the stability of the platform, an “importance factor” has been considered for each critical member based on its location and orientation in the structure, and then the reliability of the whole structure has been obtained by combining the reliability of the critical members with its specific importance factors.