Objective To evaluate the grey-scale and color Doppler ultrasound on differentiating benign and malignant serous cavity fluids. Methods To study the characters of grey-scale and color Doppler ultrasound on differentiating benign and malignant serous cavity fluids, and score according to the probability of malignancy. The main diagnostic indicators were screened out by Logistic regression model and then got the formula which could calculate the probability of malignancy. Meaningful indicators of malignant serous cavity fluids were thickness of gall bladder wall, greater omental thickening/nodules, intestinal wall thickening, blood flow of the greater omentum, bowel floating and retroperitoneal lymph node. The following formula were obtained by two-variable logistic regression model: Probability of malignancy = 1/ (1+e-z), which z=4.226×thickness of gall bladder wall +5.218× greater omental thickening/nodules +3.024× intestinal wall thickening +4.892× bowel floating -29.387. The OR of the greater omental thickening/nodules was higher than other independent variables. If serous cavity fluids with thin gallbladder wall, greater omental thickening/nodules, intestinal wall thickening and bowel floating, the probability of malignancy was 0.99. Results The thickness of gall bladder wall, greater omental thickening/nodules, intestinal wall thickening and bowel floating were effective indicators to predict malignant serous cavity fluids by ultrasound, and the established Logistic regression model has a higher value inSdifferentiating benign and malignant serous cavity fluids. Keywordserous cavity fluids, Logistic regression model, benign and malignant