ABSTRACT In this work two concentric capillary tubes-lense interferometers have been used with the same laser focused sheet of light. One of the lense-capillary tube interferometers was filled with the crude oil under investigation, while the second lense-capillary tube interferometers was filled with air at room temperature and normal pressure. The transverse interference fringes produced from each of the interferometers were allowed to be combined together displaying the Moiré interference fringe pattern that can be used for the calculations of the crude oil refractive index and density. Also PC software was performed to extract the information needed from the resulting Moiré pattern to obtain the API gravity of the oil. (American Petroleum Institute)