Alzheimer's disease (AD) is a neurodegenerative disease associated with memory and cognitive dysfunction and is the main reason of dementia worldwide. The present research aimed to evaluate the protective effect of administration of probiotic-fermented milk and costus extract against AD in rats. Total phenolic content (TPC), total flavonoids (TF) and antioxidants activity (AA) of costus extracts were studied. The study included 40 rats divided into five groups. Group 1 and 2 served as normal control and AD control, respectively. Groups 3 and 4 were given daily oral dose of costus extract (100 and 200mg/KG RBW), while group 5 was given probiotic-fermented milk (1 ml/rat/day). AD was induced in rats by intraperitoneally (i.p.) injection of D-galactose and aluminium chloride for 21 days. Hippocampal gene expression of PI3K and Akt, biochemical, nutritional and histopathological studies were evaluated in all rats' groups. Microflora of fecal samples from cecum and colon were studied in group 1, 2 and 5. Costus root extract showed 98% AA and contains TPC and TF as 378.1 mg GAE/g and 275.3 mg/g extract, respectively. Induction of AD in rats led to reduction of mRNA level of PI3K and Akt in hippocampal of rats, also elevation of inflammatory and oxidative stress markers in association with significant elevation of acetylcholinesterase activity were observed. The results revealed relevant new visions towards the influences of probiotic-fermented milk and costus extract on cognitive function through reduction of neurofibrillary tangles formations, regulation of hippocampal gene expression of PI3K and Akt, inflammatory and oxidative stress markers. Conclusion: Administration of probiotic-fermented milk and costus extract have a promising effect on cognitive function through reduction of neurofibrillary tangles formation, regulation of hippocampal gene expression of PI3K and Akt, inflammatory and oxidative stress markers. The present results suggested that intake of probiotic-fermented milk had a beneficial effect on the composition of cecum and colon flora. Probiotic-fermented milk was the most effective for prevention of AD in the present study.