Tanshinone IIA is one of major constituents of Salvia miltiorrhiza Bunge known as Danshen. Our and others' studies have shown that Tan IIA could protect cardimyocyte against apoptosis; however the effect of Tan IIA on cardiac remodeling disease is still unknown. In this study, we investigated the effects of Tan IIA on cardiac hypertrophy and fibrosis in two-kidney, two-clip (2K2C) hypertensive rats and by which, if any, mechanisms. Administration of 2K2C hypertensive rats with Tan IIA attenuated cardiac dysfunction and fibrosis. However Tan IIA treatment had no effects on BP control. Further studies revealed that Tan IIA inhibited the increased NAD(P)H oxidase activity and expression as well as O2*- production in 2K2C hypertensive rats. Our results indicated that Tan IIA significantly improved cardiac function and attenuated fibrosis in 2K2C hypertensive rats. The protective action of Tan IIA is likely mediated by its antioxidant effect, independent of BP control, partially via inhibiting NADPH oxidase.