Abstract： Objective:To analyze the MRI characteristics of surgical resected renal angiomyolipoma (AML) smaller than 4 cm.Methods:A total of 112 patients with surgical pathology confirmed renal AML of which the maximum diameter was smaller than 4 cm were analyzed retrospectively in the First Medical Centre, Chinese PLA General Hospital from January 2014 to November 2020, 5 of which were epithelioid angiomyolipoma (EAML) patients. According to the presence or absence of visible fat in lesions on MRI, the lesions were divided into AML with fat group and AML without visible fat (AML wovf) group. The MRI features were evaluated, including maximum diameter, location, growth pattern, shape, beak sign, angular interface with renal parenchyma, pseudo-capsule, hemorrhage, cystic degeneration, coagulative necrosis, flowing void in the tumor, signal intensity and homogeneity on T 2WI and diffusion weighter imaging (DWI), the peak enhanced phase. The differences of maximum diameter of AML with fat and AML wovf were analyzed using Mann-Whitney U test, and the differences of MRI features were analyzed using χ 2 test or Fisher′s exact probability test. Results:There were 123 lesions found in 112 patients, and 96 lesions contained fat and 27 lesions were AML wovf. 82 lesions showed round and round-like shapes, 112 lesions showed exophytic growth pattern, 71 lesions with peak enhancement in corticomedullary phase. And the numbers of lesions with angular interface with renal parenchyma, beak sign, cystic degeneration, pseudo-capsule, hemorrhage were 30, 49, 1, 1, 1, respectively. There was no coagulative necrosis in all lesions. Compared with AML with fat, AML wovf was single lesion. The diameters of AML with fat and AML wovf were 2.5 (1.7, 3.5) and 1.8 (1.4, 2.3) cm respectively, with statistically significant difference ( Z=-2.80, P=0.005). In the AML with fat and AML wovf, 65 and 12 cases were heterogeneous in T 2WI, 44 and 5 lesions showed beak sign, 26 and 4 lesions showed angular interface with renal parenchyma, 57 and 10 cases were heterogeneous in DWI. And there were 5 and 6 lesions showed the endophytic, 44 and 8 lesions showed partly exophytic, 47 and 13 lesions showed exophytic in patterns of tumor growth respectively. The beak sign, homogeneous in T 2WI and DWI, patterns of tumor growth showed statistical differences in AML with fat and AML wovf (all P<0.05), and there was no significant difference in other features ( P>0.05). A total of 5 EAML patients were with 8 lesions. One patient had 4 lesions with fat, other patients had single lesion in which 2 lesions with fat, 2 lesions without visible fat. One lesion without visible fat showed hemorrhage. Conclusions:Surgical resected AML smaller than 4 cm is often exophytic round and round-like, enhanced in corticomedullary phase, showing angular interface with renal parenchyma and beak sign, with rare cystic degeneration, pseudo-capsule, hemorrhage and improbable coagulation necrosis. AML wovf is single smaller lesion which often shows endophytic growth pattern, and beak sign is infrequent. EAML seems to be present in two modes, multiple lesions with fat and AML wovf with hemorrhage.