Background and Objectives: A recent study proposed simple classifications of microscopic vascular invasion (MVI): microscopic portal vein invasion (MPVI) and microvessel invasion (MI). We aim to validate these classifications of MVI. Methods: This retrospective study consecutively enrolled 514 Barcelona Clinic Liver Cancer stage 0, A, and B naïve hepatocellular carcinoma patients who underwent liver resection in our institution from 2011 to 2017. Results: Among these 514 patients, 240 patients were classified as having no MVI at all (designated as no vascular invasion, NVI), 157 patients were classified as having MI only, and 117 patients were classified as having MPVI. The 5-year overall survival (OS) rate in the MI-only group was 83.3%, which was not significantly different from that of the NVI group (87.2%), p = .20. Using NVI as a reference, multivariate analysis showed that MI-only is not an independent variable associated with OS. The 5-year OS in the MPVI group was 59.2%, which was significantly lower than those for MI-only (p < .001) and NVI groups (p < .001). Using NVI as a reference, multivariate analysis showed that MPVI is an independent variable associated with OS (HR, 3.12; 95% CI, 1.80-5.40; p < .001). Conclusions: The results of this study validate the simple MVI classifications to be clinically useful. K E Y W O R D S microscopic vascular invasion, overall survival, liver resection, hepatocellular carcinoma 1 | INTRODUCTION Hepatocellular carcinoma (HCC) is a common cause of cancerrelated deaths worldwide, including Taiwan. 1 Liver resection (LR) is one of the widely applied treatment modalities for HCC. 2 Microscopic vascular invasion (MVI) is a powerful predictor of poor overall survival (OS) after LR for patients with HCC. 3,4 Mild MVI may consist of a few malignant emboli located near the primary tumor, whereas advanced MVI may consist of multiple, large foci of malignant emboli scattered throughout the liver. The outcome should theoretically be worse in cases of advanced MVI. 5 Numerous classifications for MVI have been proposed. These classifications include those based on the number of vessels with MVI (≤5 vs. > 5), the number of cancer cells within a vascular lumen