Aberrant accumulation of lipid droplet (LD) is a main feature of foam cells constitutes a hall marker of atherosclerosis. Therefore, understanding the physiological behaviors of LDs shows an important end-point analysis for drug discovery in atherosclerosis. Herein, based on optical diffraction tomography, we performed quantitative biophysical analysis of live foam cells in label-free manner. By co-localization of specific refractive index (RI) value with fluorescence of Nile red, we identified RI of LDs distinguished from foam cells. Through time-lapse monitoring, validation of therapeutic efficacy for a targeted nanodrug was presented by biophysical parameters. Furthermore, using machine-learning based image analysis, we further demonstrated therapeutic effect at the single-cell level. These results suggest that measurement of RI is a promising method to develop new drugs against LD related diseases.