Introduction: The cerebrovascular complication of cardiac cancer can be one of rare causes of seizures in patients with neoplasms. However, primary cardiac tumors are a rare entity in children, and the prevalence is 0.027 to 0.08%. We report a boy diagnosed with cardiac fibrosarcoma presenting with seizure and hemiparesis. Case: A 7-year-old boy visited our emergency department presenting with sudden onset seizure of right facial twitching and right extremities. Initial magnetic resonance imaging (MRI) showed venous engorgement involving the left hemisphere. Electroencephalography (EEG) had slow background activities over the left hemisphere. He had drowsy mentality following seizure, so was initially presumed with focal encephalitis. His right extremities were significantly hypotonic with the strength of grade 1 to 2/5 two days later since the first seizure. Follow-up MRI with angiography revealed acute infarction consistent with the left middle cerebral artery and multiple embolisms. Through the comprehensive examination for the causes of childhood stroke, a huge mass in mitral valve was found on echocardiogram. He was finally diagnosed with cardiac fibrosarcoma by cardiac biopsy and cytogenetic study. After that time, he started the medication of chemotherapy until now. Conclusion: Cardiac cancer in children is very rare and its outcome is poor. Pediatric physicians essentially must evaluate the cardiac etiology when they confront a child showing seizure with stroke sign.