The differentiation of cardiac fibroblasts (CFs) into myofibroblasts and the subsequent deposition of the extracellular matrix is associated with myocardial fibrosis following various types of myocardial injury. In the present study, the effect of curcumin, which is a pharmacologically-safe natural compound from the Curcuma longa herb, on transforming growth factor (TGF)-beta 1-induced CFs was investigated, and the underlying molecular mechanisms were examined. The expression levels of alpha-smooth muscle actin (SMA) stress fibers were investigated using western blotting and immunofluorescence in cultured neonatal rat CFs. Protein and mRNA expression levels of alpha-SMA and collagen type I (ColI) were determined by western blotting and reverse transcription-quantitative polymerase chain reaction. In addition, the activation of Smad2 and p38 was examined using western blotting. Curcumin, SB431542 (a TGF-beta R-Smad2 inhibitor) and SB203580 (a p38 inhibitor) were used to inhibit the stimulation by TGF-beta 1. The results demonstrated that the TGF-beta 1-induced expression of alpha-SMA and ColI was suppressed by curcumin at the mRNA and protein levels, while SB431542 and SB203580 induced similar effects. Furthermore, phosphorylated Smad-2 and p38 were upregulated in TGF-beta 1-induced CFs, and these effects were substantially inhibited by curcumin administration. In conclusion, the results of the present study demonstrated that treatment with curcumin effectively suppresses TGF-beta 1-induced CF differentiation via Smad-2 and p38 signaling pathways. Thus, curcumin may be a potential therapeutic agent for the treatment of cardiac fibrosis.