Objective To study the relationship between the washout speed in the contrast-enhanced ultrasound (CEUS) and the differentiation grading in pathology in patients with hypervascular hepatocellular carcinoma (HCC).Methods Totally 271 patients who underwent complete resection for hypervascular HCC from April 2009 to December 2014 at Nanchong Central Hospital were included in this study.CEUS examinations were perfomred in all patients 2-3 days before liver resection.A timer on the ultrasound screen displayed the time elapsed since the saline flush was used to determine time to washout.The washout rate were categorized into four levels based on enhancement in portal venous phase and delayed phase.The differentiation grade and the washout speed were compared using Kruskal-Wallis test.Results All 271 (100％) lesions were arterially enhanced with different washout rate.Washout speed 1 was found in 19 patients (7.0％),while speed 2 in 157 patients (57.9％),speed 3 in 65 (24.0％),and speed 4 in 30,respectively.The washout speed had a significant correlation with the differentiation (x2=179.8238,P ＜ 0.001).The faster washout speed,the higher differentiated.Washout speed 1 could distinghuish well-differentiated from the poor and moderately differentiated tumor.The sensitivity and specificity of preoperative washout speed 1 to identify well histologic grade were 98.0％ and 77.8％,the positive predictive value and negative predictive value were 96.0％ and 48.8％,and the positive likelihood ratio and negative likelihood ratiowere were 1.9 and 0.01.Washout speed 4 could distinghuish poor differentiated from well and moderately differentiated tumor The sensitivity and specificity of preoperative washout speed 4 to identify poor histologic grade were 24.3％ and 97.0％,the positive predictive value and the negative predictive value were 65.3％ and 61.3％,and the positive likelihood ratio and negative likelihood ratiowere were 3.2 and 0.7.Conculsions Washout speed can reflect the degree of differentiation roughly.Washout speed can effectively diagnose the well-differentiated HCC,but it is difficult for moderate and poor differentiated HCC.