Objective To investigate the effects of parenteral nutrition with ω-3 fatty acids on nutrition,inflammatory response,immunity and prognosis in criticallyill cancer patients.Methods 60 critically ill cancer patients were randomly divided into study group and the control group,30 cases each.All Patients were given isonitrogen and isocalorie enteral and parenteral nutrition for 2 weeks,which added ω-3 fatty acids in the study group by parenteral nutrition.The indicators of nutrition,inflammation,immune function were detected weekly.prognostic indicators (mortality,intensive care unit [ICU] length of stay,infectious complications) were compared.Results The indicators of nutrition,inflammatory reaction and immune function of study group were significantly improved than control group,and length of stay in ICU of study group was shorter than control group.There were less mortality and morbidity of infection complications in the study group compared with control group,but difference were not significant (P =0.13 and P=0.165).Conclusion ω-3 fatty acids supplementation of parenteral nutrition in critically ill cancer patients can improve nutritional status and immune function,reduce inflammatory response,shorten ICU length of stay,but not significantly improve ICU mortality and reduce infectious complications.