The temperature dependence of electrical resistivity has been measured for pure iron with various RRR_H,4.2K values from 352 to 11390. It is found that the experimental data in the temperature range from 1.6 K to room temperature can be expressed by a simple sum of the phonon scattering term, a term proportional to T² and residual resistivity. The phonon scattering term is estimated by the Grüneisen-formula. The coefficient of the T² term is (2.2±0.1)×10⁻¹¹(Ω·cm·deg⁻²) and the term proportional to T is negligibly small. A new index, RRR_H,0K, is suggested as an appropriate measure of purity for highly purified iron, and the largest value of RRR_H,0K obtained is 20210 on the sample with RRR_H,4.2K 11390.