Objective To study the pathological features of two huge spontaneous tumors in Wistar and GK rats.Methods Forty Wistar rats and 40 GK rats were included in this study. Among those rats, two huge spontaneous tumors were observed in a Wistar rat at 14 months of age and in a GK rat at 22 months of age. The growth and survival status of the tumor-bearing rats were recorded. The tumors were surgically removed, and their pathological features were examined using HE and immunohistochemical staining (vimentin, CK19, α-SMA, CD31, CD34, S-100, NF及Ki-67). Results Both the two tumors were completely resected by surgery without much difficulties, and both host rats survived well after the operation. The weight of the two huge tumors was 502 g and 119 g, which corresponding to 64% and 24% of the body weight of their host rats, respectively. The tumors surface had a complete capsule, with a clear boundary separating from the normal surrounding tissues, and no vascular pedicle structure was found. According to the results of immunohistochemical staining, both the two tumors were diagnosed as benign fibroma. Conclusion This type of huge spontaneous tumors is benign fibroma. Besides the impact on the activities of the rats, the tumors have no significant impact on the living conditions in the hosts.