Mechanical analysis of a three phase oil separator had been done previously without wind and earthquake load yet. Therefore this research is focused on the mechanical analysis that takes into account the wind and earthquake loads. The mechanical analysis follows ASME Code Section VIII and was conducted using graphical based software. Since ASME standard code does not provide specifically construction code on wind and earthquake load then it follows the standard of ASCE 7-2005 for wind load and ASCE 7-98/02/ IBC 2000 for earthquake load. Wind load was determined base on wind speed pressure, gust-effect factor, force coefficient and projected area normal to the wind. Earthquake load depends on intensity and duration of earthquake. Analysis was carried out on the head, shell, nozzle and skirt of the vessel though wind and earthquake load effect the skirt only. The objectives of this research are to determine the vibration possibility and static deflection due to the wind load and allowable stress due to earthquake load on the vessel design. The result shows that the pressure vessel in a stable condition and the static deflection is less than the allowable deflection. The pressure vessel also meets the criteria of stress due to earthquake load. Keywords : Oil separator, pressure vessel, wind load, earthquake load, graphical-based software