A convenient plotting method in the Archibald method generally adoptable for determining the molecular weight of proteins was developed. This method consists mainly of plotting of l/x-(dn/dx)_xt vs. c_xt, which is designed to read the exact slope, l/x,-(dn/dx)_xt and to exclude an error arisen from the separate run with synthetic boundary cell necessary to estimate the initial concentration of the solute. The experimental results indicated that this convenient method provided the same accuracy for the determination of molecular weight as that of usual Archibald method, when the initial concentration of the solute was suitably assumed and used in the calculation of thr provisional or supposed concentation of the solute at any position in the cell of ultracentrifuge.