Lupus nephritis is a common and serious manifestation of systemic lupus erythematosus,its diagnosis is established by a combination of clinical ,laboratory, serological data and histology of the kidney. Plasma complement 4 d can be used to predict flare of lupus nephritis. Aim: The aim of our work was to assess the relation between plasma complement 4 d and lupus nephritis.. Methods: This study was carried out on 100 Lupus Nephritis patients fulfilled the Systemic Lupus International Collaborating Clinics (SLICC) classification criteria and 100 apparently healthy age & sex matched controls. Plasma complement 4 d was detected by specific sandwich ELISA according to the instruction of the manufacturer (SunRed, Shanghi). Results: Our work showed that 24 hour urine protein,RBCs in urine, serum albumin, pus in urine were (significantly associated) with plasma C4d (unstandardized β=0.001, -0.213, 0.441, 0.041 and 0.024 respectively) (p < 0.05). Our results also revealed that there was statistically significant relations between plasma c4d and lupus nephritis in SLE patients (p < 0.05). Conclusion: The study evaluated the importance of Plasma complement 4d in lupus nephritis and confirmed that plasma complement 4d can be used as a very good marker of the flare of lupus nephritis.