The each quantitative determinations of La and Ce are difficult. It is recommended to measure the quantities of La and Ce from their mixed solutions with the method of co-measurement by using electro photometer，giving the±4% error in the resion of 0～4000γLa^<3＋>or Ce^<3＋>. And the treatment to remove the disturbance by Al^<3＋>, Fe^<3＋> and Cr^<3＋> are given. The mathematical generaligation of Job's continuous variation method is also tried.