To determine lethal temperature of orchardgrass (Dactylis glomerata L. cv. Janbeol 102) developed in Korea at heat-stressed conditions, seedlings grown in a amall pots for 4 weeks were treated at $45^{circ}C$, $50^{circ}C$ or $55^{circ}C$ for 1 h. Heat treatments at $60^{circ}C$ and $65^{circ}C$ for 1 h, several plants were withered and showed damage symptom on their leaves. When the plants were exposed to $70^{circ}C$ for 1 h, most of leaves were severely withered, but it was not lethal conditions for the whole plants. By contrast, most of plants were died within one day after heat treatment at $80^{circ}C$ for 1h. Furthermore, plants exposed to $80^{circ}C$ for 55 min were also died within 7 days. It was found that new shoots were regenerated from the plants that had been treated at $80^{circ}C$ within 50 min. These results indicate that heat treatment at $80^{circ}C$ for 55 min is an optimum condition to distinguish the lethality of orchardgrass plants. Simple viability assay system established in this study will be useful for selection and characterization of heat-tolerant transgenic orchardgrass plants.