Abstract: The influence of the perturbation on crystal growth patterns of plane and cell at the solid/ liquid interface during unidirectional solidification has been studied. The critical condition of the transition from plane to cell of alloys and the condition of the steady state growth of cell are given. It has been demonstrated by linear steady theory of nonlinear theory for the first time that the bifurcation mechanism of the transition from plane to cell and the interface of cell of alloy at the solid/liquid interface during unidirectional solidification is subcritical bifurcation. This bifurcation mechanism is proved by BrC₄ alloy.