In this study, surface positive charge intensity of a polyethylenimine (PEI) modified nanofiltration (NF) membrane was characterized by adsorption of counter ions WO42- on the membrane surface at pHs 2-10 followed by quantitative analysis using X-ray photoelectron spectrometry (XPS). Retentions of NaCl and two representative antibiotics (cefadroxil and enrofloxacin) by this membrane were systematically evaluated at pHs 2-10. It was found that the PEI (25,000 g center dot mol(-1)) modified NF membrane exhibited positive charge and the charge intensity was strong, moderate and weak in the pH range of< 4, 4-7, and 8-10, respectively. Such surface charge characteristics at different pH had significant effect on separation performance of ionic species such as NaCl and antibiotics. Donnan exclusion mechanism played a critical role in NaCl retention across different pH. The electrostatic interaction between the dissociated species of antibiotics and the membrane surface governed the retention behavior at various pH conditions. The orientation of the polar antibiotic molecules may be induced by the membrane surface charge, which could decrease retentions. This study may provide fundamental understanding of positively charged NF membrane application in removal of trace organic contaminants.