Background: Host immune system response against blood group antigens is a major problem in blood transfusions, especially for thalassemic patients. Thus, an approach was proposed coating the red blood cell (RBC) surface by polyethylene glycol. Objectives: This study aimed to obtain the optimal simultaneous camouflage of the major and minor antigens by activated methoxy polyethylene glycol (mPEG) with succinimidyl valerate (SVA) and succinimidyl carbonate (SC), separately. Materials and Methods:The degree of RBC agglutination by antibodies against the major and minor blood groups was used as a surrogate measurement for quantitative assessment of the effectiveness of the surface coating. Also, the RBC morphology was assessed using scanning electron microscope (SEM). In addition, to evaluate the host immune system response, the PEGylated RBCs were transferred between two different mouse strains. Results: Statistical analysis of the results demonstrated that the optimal reaction conditions for simultaneous coating of the antigens by mPEG-SVA and mPEG-SC are as mPEG 20 in the polymer mixture, 91.2 and 90.0%, and polymer concentration, 17.21 and 19.80 mg.mL -1 , respectively. However, according to the SEM results, the maximum polymer concentration of 14.5 mg.mL -1 was suggested as the best condition for mPEG-SVA modified human RBCs. Conclusions: It is concluded that the membrane PEGylation camouflages the blood group antigens. This effect is observed significantly for non-ABO/Rh(D) antigens. Also, it is found that the mPEG-SVA provide better coverage than mPEG-SC. The results of in vivo analysis showed that the immune reactions against PEGylated RBCs were considerably reduced, so that the levels of the relevant biochemical parameters in serum were similar to those of the normal hosts 24 hours after transfusion.