The mechanical degradation of poly (-vinylacetate) in several mixed solvents (dioxane-n-butanol, dioxane-sec-butanol, dioxane-ethyleneglycol, dioxane-kerosene.) was studied using the capillary flow method. The velocity constant of scission reaction (k) and the limited degree of polymerization (g) were compared at the same value of [${eta}$], which is considered as the parameter of molecular dimension of polymers in solution. As result, (k) did not change much, even if the species and the volume fraction of poor (non-) solvents changed, while the value of (g) changed according to the species of poor solvents and the value of [${eta}$]. From the facts described above, It follows that the limited degree of polymerization (g) were affected by the composition and distribution of mixed solvent molecules around the polymer chain, and the value of $alpha$ (at [${eta}$] = $KM^a$) in the polymer solution.