The instantaneous response of a laminar diffusion flamelet is investigated numerically. A detailed mechanism GRI-Mech 3.0(53-species and 325-reaction) is empolyed to describe the CH₄ oxidation and NO_x formation. A predication of steady flamelct structures is compared with the experimental data to validate the method. A step variation of strain rate is used to simulate the influence of a instantaneous flow field on the flamelet local structure. The response of flamelet structures(temperature and species concentration) to the strain rate variation is given and analyzed. We focus on the influence of the step size of the strain rate. It is found that the flamelet response to the strain rate variation is not symmetric,and the response time is inverse proportional to the strain rate variation as it is small. In addition, the mean response time of temperature is much longer than the flow time scale of a typical turbulent combustion field, which demonstrates the importance of unsteadiness in the numerical simulation of turbulent combustions.