Background oriented schlieren (BOS) is a new technique for flow visualization. Compared with schlieren and interferometry, BOS can measure density gradient quantitatively with high spatiotemporal resolution. Based on its principle, a BOS system is set up in this paper. Using this system, the thermal convection flow field over a candle flame is measured, and its transient flow structure is visualized. The whole process verifies the performance of the BOS system. Then, a supersonic mixing layer is studied, its fine flow structures and quantitative density gradient distribution are acquired. The results show obvious advantages of BOS in flow measurement.