Abstract: OBJECTIVE To study the protective effect and mechanism of grape seed proanthocyanidins(GSP) on phenylephrine(PE)-induced cardiac hypertrophy in neonatal rats. METHODS A model of neonatal cardiac hypertrophy was established by PE(20 μmol·L^-1) stimulation for 48 h, which was divided into normal control group, model group, GSP low, medium, and high dose group(10, 20, 50 μmol·L^-1). After 48 h of continuous administration, Image J was used to measure the area of cells, Western blotting was used to detect the expression of atrial natriuretic peptide(ANP) and β-myosin heavy chain(β-MHC). Subsequent experimental groups:normal control group, model group, GSP negative control group, GSP medium dose group, fluorescent probe DCFDA was used to detect reactive oxygen species(ROS) content, kits were used to detect intracellular superoxide dismutase(SOD) activity, malondialdehyde(MDA) content, lactate dehydrogenase(LDH) activity of supernatant, Western blotting was used to detect RhoA protein and ROCK activity. RESULTS Compared with control group, GSP intervention could significantly reduce the area of cardiomyocytes(P<0.05), reduce the expression of hypertrophic genes ANP and β-MHC(P<0.05), and reduce the fluorescence content of ROS(P<0.05), improve SOD activity, reduce MDA content and LDH activity(P<0.05), and significantly inhibit RhoA protein and ROCK activity(P<0.05). CONCLUSION The protective effect of GSP on cardiac hypertrophy is related to the inhibition of ROS and RhoA/ROCK signaling pathway.