Amino acids are crucial constituents of plant proteins as they serve as the fundamental building blocks. Obtaining the amino acid profile is essential for evaluating the nutritional value of plant protein. This chapter outlines a clear procedure for accurately quantifying the absolute content of 17 amino acids in plant protein. The process involves hydrolyzing the solid protein sample with a hydrochloric acid solution, followed by derivatizing the amino acids using 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate. The derivatized amino acids are then separated using reversed-phase high-performance liquid chromatography with an ultraviolet detector. The absolute quantity of each amino acid is calculated based on the internal standard and amino acid standard curves. This procedure utilizes common instruments and chemicals, and it involves a straightforward operation, providing high accuracy and good repeatability. Detailed explanations of experimental steps and calculation methods enable easy replication of this procedure by other researchers.