In this article, dynamical features of non - equilibrium phase transitions are dis-cussed, taking the physical problems of OPO and laser processes as two examples. The phase transition point is found to be the point of sudden change from a stable nodal point to a saddle point. The role of fluctuations and dissipations in such phase transi-tions is elucidated, and critical exponents analogous to those in the second order phase transition are found (β = 1/2, γ = 1).