Neurodegenerative diseases are significant health concerns with regard to morbidity and social and economic hardship around the world. Many researches in the field of Alzheimer disease have validated the CSF Tau level especially phosphorylated tau (P-tau) in the prediction of conversion of mild cognitive impairment (MCI) to clinically definite Alzheimer disease. Objectives: To study plasma tau level in patient with Alzheimer's disease and elderly patients undergoing normal aging degenerative changes Patients and Methods: The study enrolled 50 patients with symptomatic AD, or with memory impairment in the consensus of mild cognitive impairment (MCI). They were subjected to cognitive scales and plasma tau assessment. Results: Plasma tau levels assessment revealed that all patients in Alzheimer group had high level of plasma tau and MCI group nearly half of them (47.3%) had high levels meanwhile the control groups did not revealed and high levels of plasma tau, these findings revealed high statistical significance (ρ-value <0.001). Higher levels of plasma tau are consistent with low MMSE scores, MoCA scores and high CDR scale score with high statistically significant differences (ρ-value <0.001). Conclusion: Plasma tau levels can give an overview of pathophysiological changes at the cellular level in patients with Alzheimer's disease, which increases the biomarker's diagnostic value. Achieving a reliable biomarker with adequate sensitivity and specificity is necessary in detecting AD at early stages