PD-1/PD-L1 inhibitors activate immunological response and have become one of the main modalities of cancer treatment. However, they may result in the immune-related adverse events (irAEs). Immune-related cardiotoxicity is relatively rare but may become fatal. We will present a case of a male patient who experienced immunotherapy-related cardiotoxicity one year after received pembrolizumab treatment. The patient had atypical symptom presentation initially, but his condition deteriorated worsened rapidly and he developed severe cardiac disease. The patient experienced significant relief after corticosteroid treatment. Unfortunately, he experienced a reoccurence of the severe adverse event when discontinuing the use of corticosteroids. Ultimately, larger doses and longer courses of corticosteroid treatment cured the heart damage. Fortunately, we observed that lesions were stable and maintained for a long time after cessation of using pembrolizumab for eight months.