Abstract: Objective To analyze the morphological features of omental milky spots (MS) in peritoneal carcinoma. Methods Hematoxylin-eosin (HE) staining and immunohistochemistry (IHC) were used to analyze the omental MS in peritoneal carcinoma (PC) patients. We focused on the morphological features of MS and conducted quantitative analysis on their cells number and cellular constituent. Meanwhile, the differences of MS parameters were analyzed. Results Various shapes of MS were mainly round (14.5%), oval (18.0%), irregular form in the adipose (14.0%) and perivascular annulus (11.5%). The median perimeter of MS was 2752 (range 817-7753) computer-based pixels. The median value of immune cells in one MS was 141(43~650). Immune cells were the main component of MS, comprising T lymphocytes (46.1%), B lymphocytes (28.4%), macrophages (12.4%) and other immune cells (13.1%). Relatively high density of blood vessels in MS could be calculated by micro-vessel density (MVD) as 4(0~13). The median number of mesothelial cells loosely arranged in the surface layer was 5(0-51). The percentage of MS without surface mesothelial cells was 38.1%. The total number of immune cells, perimeter, MVD, the number of mesothelial cells, T lymphocytes and macrophages in omental MS were not significant different between rectal cancer patients and gastric cancer patients (P>0.05), while the number of B lymphocytes was significantly different (P<0.001). Conclusion Milky spots are the primary immune tissues of omentum and the structural bases for the development and progression of PC. Analyzing the morphology and cellular constituent promotes studying the pathomechanism of peritoneal carcinoma.