The degree of polymerization of the PMMA contained in the silk fibre, which was obtained by graft copolymerization by using Cu(II)/H$_2$O$_2$as an initiator was studied under the various conditions of polymerization. For both the grafted PMMA and free PMMA formed out side the silk fibre, the degrees of polymerization were found as follows. (1) The degree of polymerization was roughly proportional to the concentration of monomer. (2) With increasing concentration of cupric salt, the degree of polymerization was decreased gradually. (3) The effect of hydrogen peroxide on the degree of polymerization was considerable, and D.P. has a maximum value at concentration of 10-$^1$mole/l and the curve of degree of polymerization was similar to that of graft-on. (4) The maximum value of graft-on and that of the degree of polymerization were at 30 min. 70$^{circ}C$. The value of graft-on after 30 min. was steady, however, the degree of obtained polymerization was decreased. (5) The degree of polymerization was affected considerably by the temperature of the polymerization and its maximum value was attained at 70$^{circ}C$ as in graft-on. (6) The degree of polymerization of the graft PMMA is generally larger than that of the free PMMA outside of the fibre.