Abstract: We demonstrate the observation of gold-nanoparticle internalization in membranes of living cells by using noninterferometric widefield optical profilometry (NIWOP). The NIWOP technique can trace the height of an 80 nm gold particle on the membrane by calibrating the change of light intensity scattered from the particle along the optical axis. On the membrane, the depth resolution based on the scattering signal is similar to that based on the reflection signal, nearly 20 nm. Comparing the heights of the nanoparticle and the nearby cell membranes, we can identify the occurrence of particle internalization. Combining fluorescence microscopy with NIWOP, we also find actin aggregation around the site of the internalization process, which is an indication of endocytosis.