In the present work, the topographical images of cavitation erosion surfaces at water and oil-in-water (o/w) emulsions were quantified using fractal analysis. The oil-in-water emulsion concentrations were 2, 5 and 10 wt. %, which are in the popular range used in hydraulic systems. The study showed that the variation of fractal dimension calculated from slope of linearized power spectral density for water and o/w emulsions can be used to characterize the cavitation intensity in similar manner to the weight loss. Both the fractal dimension and the weight loss decrease with adding oil to water. It was also found that the variation of fractal dimension versus concentration of oil-in-water emulsions has a general trend that does not depend on magnification factor. The cavitation erosion behavior and mechanism for water and o/w emulsions has been studied. It was found that the predominant failure mode was fatigue for water and o/w emulsions.