The present invention relates to a method for diagnosing a disease using an analysis of oligomer of an abnormal aggregated protein includes: (1) preparing a body fluid sample including at least one of blood, blood plasma, blood serum, saliva, urine, tear, and mucus; (2) making a dilution of the body fluid sample; (3) using a biosensor to measure and detect an aggregated protein in the diluted body fluid sample; (4) analyzing a signal change of the biosensor caused by the dilution of the aggregated protein to determine a slope according to the dilution from the measurements; and (5) analyzing a proportion of the oligomer from the slope according to the dilution to make a diagnosis. The method uses a biosensor to measure the impedance and the protein concentration of blood and detects the slope according to the numerical value of the monomer and the oligomer to diagnose normal or abnormal protein aggregation or the associated diseases with more accuracy.