The unsteady behavior of diffusion flames was experimentally studied with a jet diffusion flame under a configuration similar to counterflow. We designed and conducted our experiments in such a manner that the fuel was ejected downward so that various time histories of strain rate with good reproducibility were obtained. The fuel jet was ignited by means of a residual flame sustained at the nozzle tip, and the developing process was visualized by both Mie scattering and high-speed shadow imaging. The maximum flame temperature was measured by a compensated thermocouple on the flame tip. A new method was adopted for the time constants. The results show that the history of the strain rate has a significant role in the unsteady behavior of laminar diffusion flames, especially close to flame extinction. Regarding of quenching histories, the extinction strain rates extend as the slope of the strain rate increases, which has not previously been explored experimentally. Even in the non-quenched flame, an unsteady effect is shown based on the fact that there are two maximum flame temperatures at the same strain rate and the one experiencing the higher strain rate is always lower.