A perturbational approach is presented to the external field effects for chemical reaction. Chemical reaction is treated by the “string model” in which the reaction path is represented by the intrinsic reaction coordinate (IRC) as a string. The IRC is defined mathematically as a unique curvilinear reaction path which connects reactant (R) and product (P) via a transition state (TS) on the potential energy surface. The string is thrown in the external field which acts as a nonadiabatic source of perturbation.