The zebrafish-based assay is a widely used animal model system for cardiovascular research. In this study, we investigated the cardiac defects caused by terfenadine and tested the pharmacological response of digoxin to zebrafish with cardiac defects. The study suggested that zebrafish could be a suitable model for phenotype-based screening and evaluation of positive inotropic agents. This phenotype-based heart failure zebrafish model system was then utilized in in-house library screen. Some positive inotropic compound was discovered, which could attenuate the cardiac defects by increasing the flow velocity of caudal artery blood.