Objective To investigate the CT manifestations of inflammatory and tumorous diseases in peritoneum, and to improve the abilities of diagnosis and differential diagnosis. Methods Conventional non-enhanced and enhanced CT were performed in 63 patients. The mean value of peritoneal structures in normal persons and patients were measured and calculated. The CT features of abnormal peritoneum were divided into 5 classes that were collated with their pathological changes. The numbers of each CT class appeared in inflammatory and tumorous diseases were countered. The data were treated with statistical methods for comparing the differences.Results (1) In normal persons, the non-enhanced and enhanced mean CT values were (-88.60?18.30) and (-78.73?16.90) HU, respectively, while in patients, they were(-38.63?15.64) and (-1.42?14.42) HU, respectively. CT value of abnormal peritoneum was significantly higher than that of normal group. (2) There were 183 places of abnormal peritoneum in five classes: firstly, fine and blur attenuation in 65 places (56 inflammations and 9 tumors); secondly, linear or strip thickening of peritoneum in 25 places (12 inflammations and 13 tumors); thirdly, attenuation of disorganized strip and piece or nodus in 50 places (8 inflammations and 42 tumors); fourthly, soft tissue mass in 28 places (5 inflammations and 23 tumors); and fifthly, cake peritoneum in 15 places ((2 inflammations) and 13 tumors). The data showed statistical differences (P