The heat denaturation process of myosin and the effect of the modification of amino groups of myosin with β-naphthoquinone-4-sulfonic acid (HQS) on heat denaturation reaction were studied by using SDS polyacrylamide gel electrophoresis.Heavy chain and one of the light chains, g2, disappeared in the supernatant after centrifugation of heat treated myosin during two hours’ heating, whereas two sorts of the light chains, g1 and g3, were still found in the supernatant. In addition, a new peptide fraction appeared during heating.Heat denaturation of myosin was significantly suppressed by the modification of amino groups of myosin with NQS, where only heavy chain’s electrophoretic pattern was influenced by NQS-modification.