The polymerization of acrylamide in aqueous solution was carried out with β-irradiation facilities using unsealed 85Kr gas as an internal radiation source. The effects of various factors on the degree of polymerization, as well as the molecular weight distribution were studied. The experimental results are compared with the results using γ-rays from a ¹³⁷Cs source. Some of the results obtained are as follows. (1) The degree of polymerization, for either source, is inversely as the 0.27th power of dose rate and proportional to the 1st power of monomer concentration. (2) In addition, the temperature dependence of the degree of polymerization shows a similar tendency. (3) On the other hand, for the molecular weight distribution, a slight difference is observed.