Apoptosis has now established its importance in numerous areas of biology and is recently receiving great attention as an important topic related to the development of diseases. In this work, an integrated microfluidic device was developed to characterize doxorubicin-induced apoptosis in human hepatocellular carcinoma (HepG2) cells. A continuous concentration gradient of stimulator (doxorubicin) was generated in the upstream network and used to perfuse downstream cultured HepG2 cells. The appropriate fluorescent dyes were introduced into cells from the inlets connected to the cell culture chambers, allowing one to distinguish apoptotic cells from nonapoptotic or necrotic cells. The resultant fluorescence of cellular population was monitored and quantified with single-cell resolution to infer the apoptosis process being studied. The feasibility of studying apoptosis was demonstrated by measuring several apoptotic events, including morphological alterations, plasma membrane phosphatidylserine externalization, and mitochondrial membrane potential collapse. This microfluidic device, integrating the cell culture, stimulation, staining, and washing steps into a single device, can simultaneously generate a number of experimental conditions and investigate multiple parameters relating stimulation to apoptosis. It offers a unique platform to characterize various cellular responses in a high-throughput fashion, which is otherwise impossible with conventional methods.