One of the important items in offshore structure design is fatigue analysis. Because of existence of periodic loads derived by wave, sea current, wind, etc. this analysis has become one of the major analysis in basic design. Estimation of structure’s life that is generated from fatigue analysis is more important in operation of the offshore unit. In this paper according to the environmental conditions of Caspian Sea, a jack up was modeled by use of ANSYS. According to available statistics of this specified region the wave by 12 m significant height and sea current by the speed of 1 (meter/second) is exerted on the structure. API rules are considered in modeling of soil and spudcan interaction and the stiffness of the elastic springs also selected by this standard. After that, nonlinear dynamic analysis has been carried out for all directions of wave motion so that the dominant wave direction and critical joint has been detected. This critical joint has been modeled by considering the effective length of members and appropriate support condition. The design loads (axial loads, out-of- plane bending, in-plane bending) have been exerted, and Hot Spot Stress “HSS” which had calculated by API formula has provided for stress combination in order to indicate the critical node. After that fatigue analysis has been done for this node by considering S-N curves in API recommendation. Finally the roughly life of the jack up rig has been determined. In addition to that analysis the effect of fatigue damage in each wave direction has been investigated and compared by using wave-energy distribution method. In this paper we try to generate a simple method for detecting the critical joint in order to calculate the structure’s life approximately.