Design of chemical processes (CP) is usually performed in condition of some uncertainty of original physical and chemical information. In connection with this the important problem of the evaluation of flexibility of CP (the ability of CP to preserve its capacity for work) arises. Grossmann introduced the flexibility function which permits to evaluate CP flexibility. But direct calculation of the flexibility function is reduced to solving vg hard nondifferentiable and multiextremal optimization problem. In connection with this we gave an effective method of calculation of flexibility function. It is reduced to an iteration procedure on each iteration at which usual non-linear programming problem is solved.