This paper reports a simple microfabrication method for single cell culture on a digital microfluidics chip. The method is based on featuring 3D microstructure array on the dielectric layer of the chip. MDA-MB-231 cells were shown to trap in the 3D microstructure array with single cell efficiency to 20%. Based on the designed microstructures, we used Cisplatin (Cis) as model to measure toxicity to breast cancer cells MDA-MB-231 on our-chip and off-chip. The results suggest that the designed structures are effective for single cell trapping, and evaluation of drug toxicity over time.