High-pressure electrophoresis was designed to investigate the association system of proteins quantitatively. In this methodology, pressure and temperature precisely perturbs the association equilibrium of supermolecular protein assembly, whose decomposed fractions can be in-situ analyzed by native polyacrylamide gel electrophoresis (PAGE). The bands of these fractions allow us not only to unravel the elementary process of assembly formation but also to quantify various volumetric properties including thermal expansion change of each process. In this article, we report its application to the problem of amyloid protofibril formation.