Knot formation in a single polymer chain was studied by measuring chain expansion processes from a globule to a coil state. Dilute solutions of poly(methyl methacrylate) in the mixed solvent tert-butyl alcohol + water(2.5 vol %) were preserved in a globule state at 25.0 degreesC for a time period t(p) and given a fast temperature jump to the theta temperature 41.5 degreesC to measure the chain expansion process by static light scattering. The chain expansion occurred slower with increasing tp for the molecular weight M-w = 1.22 x 10(7). For the constant time period t(p) = 7200 min, the expansion process became slower with increasing M-w from 4 x 10(6) to 1.5 x 10(7). These observations disclose the knot formation in a single polymer chain.