Aim: The objective is to study the role of reactive oxygen species on cardiac regeneration and fibrotic elimination in adult zebra fish after cryo-injured model of myocardial damage. The entair experiment was carried out after the approval of institutional animal Ethical committee, Institute of Basic Medical Science, University of Madras, Chennai, India.  Materials and methods: Wild type zebra fish were subjected to induction of myocardial injury by surgical method according to the methods of (González-Rosa & Mercader, 2012). After the surgery the animals were subjected to ECG analysis and the animals which showed alteration in the ST segments were included in the study otherwise were excluded. After the confirmation of ECG the animals were grouped into 4 ei., group I- sham (subjected to surgical procedure except myocardial injury), group II – (cryoinjury: induction of myocardial injury by placing pre-cooled cryo-probe (0.8 mm) for 20 seconds, group III – cryoinjury + Quercetin 50mg/kg B/W (Lara-Guzmán et al., 2012) once in a day for the periods of 60 days and group IV- sham + Quercetin 50mg/kg B/W. once in a day for the periods of 60 days.  The animals were sacrificed at 0, 3, 7, 15, 30,45and 60 days of post operative periods by over dose of anesthesia (Tricaine) and ECG analysis were done at every time point before the sacrifice. The heart tissues were subjected to various analysis like morphology, histology, fibrosis analysis by picrosirius red staining, cell cycle analysis  by BrdU staining, estimation of ROS production, estimation of levels of antioxidants,  expression of genes involved in fibrosis (fibrin, collagen, TGFb, SMAD, αSMA, FSP1 and PU.1) and cell cycle (ccne1)  were studied by immunohistochemistry and Real-time PCR. Results: The data showed that significant increase in the ROS (p<0.001) production, reduced antioxidant levels (p<0.001), accumulation of fibrin followed by collagen replacement, significant increased in the BrdU positive cell in all the three layers of the heart (endo, epi and pericardium). Gene expression data showed increased expression of fibrotic gene (fibrinogen P<0.001, collagen 1a1 p<0.01, TGFb p<0.001, SMAD p<0,001, αSMA p<0.01, FSP1- p<0,001, and PU.1- p<0,001) and cell cycle (ccne1- p<0.001) genes in time dependent manner. But inhibition of ROS significantly represses the fibrin formation (p<0.001) in the initial days of post injury and the collagen replacement (p<0.01) also was found to be delayed.   In the histological study showed more inhibition of ROS markedly increased the accumulation of inflammatory cells in the injury site when compared to cryo-injured groups. Further, the inhibition of ROS significantly altered the expression of both fibrotic and cell cycle genes (fibrogen P<0.001, collagen 1a1 p<0.01, TGFb p<0.001, SMAD p<0,001, αSMA p<0.01, FSP1- p<0,001, and PU.1- p<0,001 and cell cycle ccne1- p<0.001). Conclusion: Taken together the present data clearly emphasized that the ROS in the regenerating adult zebra fish heart after the cryoinjury is essential for regulating the reentry of adult cells in to cell division and also for PU.1 dependent phenotypic switch of fibroblast from fibrogenesis to fibrolytic and vice versa.