A novel series of 4-phenylisoquinolones were synthesized and evaluated as c-Jun N-terminal kinase (JNK) inhibitors. Initial modification at the 2- and 3-positions of the isoquinolone ring of hit compound 4, identified from high-throughput screening, led to the lead compound 6b. The optimization was carried out using a JNK1-binding model of 6b and several compounds exhibited potent JNK inhibition. Among them, 11g significantly inhibited cardiac hypertrophy in rat pressure-overload models without affecting blood pressure and the concept of JNK inhibitors as novel therapeutic agents for heart failure was confirmed.