Introduction: Pharmacological treatment of unresectable hepatocellular carcinoma (uHCC) includes sorafenib and lenvatinib, a tyrosine kinase inhibitor, which are linked to low serum levels of carnitine and reduced skeletal muscle volume. Nowadays, atezolizumab plus bevacizumab (Atezo/Bev) combination therapy is recommended as the first-line treatment for patients with uHCC. However, the association with decreased muscle mass or cardiac function is unknown. Therefore, this study aimed to evaluate the effects of Atezo/Bev on skeletal muscle volume and cardiac function in patients with uHCC. Methods: This retrospective study included 55 adult Japanese patients with chronic liver diseases and uHCC treated with Atezo/Bev. Patients were divided into three groups according to age: middle, preold, and old. Serum levels of carnitine and cardiac function were measured before and after 3 weeks of treatment. The psoas muscle index (PMI) was measured before and after 6 weeks of treatment. Results: After treatment, the global longitudinal strain was significantly lower in the old group, whereas the PMI and ejection fraction were significantly lower in the preold and old groups. However, no significant difference in serum levels of total carnitine and those fractions with treatment in each group was found. Cardiac function decreased in the preold and old groups. Conclusion: When treating patients with uHCC by Atezo/Bev, caution should be taken in preold and old patients because they are vulnerable to decreased skeletal muscle mass and deterioration of cardiac function. Strength training and regular monitoring of cardiac function are encouraged in these groups.