Abstract: The least squares method is the simple form of the method of weighted residuals (MWR). The main feature of this method is transforming a boundary problem of partial dif- ferential equation into an equivalent problem of optimization. To illustrate the calculating procedure calculations for two isothermal and two isobaric quaternary systems were made. It showed the advantages of this method in an identical computational form for any dimensional problems and the need for the intrinsic condition only in solving the equation . So the calculation for quaternary system can be easily done on personal computer and takes less calculating time.