Background: Systemic Lupus Erythematosus is a chronic inflammatory disease that is characterized by an auto antibody response to nuclear and cytoplasmic antigens, has a relapsing remitting course .Plasma complement 4 d can be used to predict activity of systemic lupus erythematosus patients. Aim: The aim of our study was to assess the relation between plasma complement 4 d and disease activity of systemic lupus erythematosus patients. Methods:This study was carried out on 92SLE patients fulfilled the Systemic Lupus International Collaborating Clinics (SLICC) classification criteria and 92 apparently healthy age & sex matched controls, Plasma complement 4 d was detected byspecific sandwich ELISA according to the instruction of the manufacturer (SunRed, Shanghi). Results:Our results revealed that there was statisticallysignificant relations between plasma c4d and skin rashes,malar rash and hair falling(p<0.05). There washighly significant positive relation between plasma complement 4d and SLEDAI score(p≤0.001),but no significant relation between plasma complement 4 d and arthritis (p=0.456). Conclusion: Our study evaluated the role of Plasma complement 4d in systemic lupus erythematosus patients and confirmed that plasma complement 4d can be used as a useful marker of the disease activity in SLE patients.