Abstract: In this study, we carry out an experimental investigation of the behaviors as well as the mechanism of the liquid-liquid drop deformation and breakup process following an impact. With high speed photography, five distinct deformation and breakup modes are captured, for which the key factors that dominate the transition are quantitatively analyzed. The results show that similar deformation behaviors may occur for a proper combination of drop sizes, density ratios between drop and ambient fluid, interfacial tensions and free falling heights. Two non-dimensional parameters, i.e. Weber number (We) and Ohnesorge number (Oh), are calculated to estimate these effects. It is found that, similar deformation behaviors may have a strong correlation with the Weber number. After a further survey of the test range of present study (1 < We < 700, 0.001 < Oh < 0.005), it can be concluded that the deformation and breakup pattern is barely affected by the Ohnesorge number, whereas exhibits a strong dependence on the Weber number.