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In this study, a revised target model for quantifying the in vitro radiosensitivity of mung bean sprout elongation to 6-MV X-rays was developed. The revised target model, which incorporated the Poisson prediction for a low probability of success, provided theoretical estimates that were highly consistent with the actual data measured in this study. The revised target model correlated different in vitro radiosensitivities to various effective target volumes and was successfully confirmed by exposing mung beans in various elongation states to various doses of 6-MV X-rays. For the experiment, 5,000 fresh mung beans were randomly distributed into 100 petri dishes, which were randomly divided into ten groups. Each group received an initial watering at a different time point prior to X-ray exposure, resulting in different effective target volumes. The bean sprouts were measured 70 hr after X-ray exposure, and the average length of the bean sprouts in each group was recorded as an index of the mung bean in vitro radiosensitivity. Mung beans that received an initial watering either six or sixteen hours before X-ray exposure had the shortest sprout length, indicating that the maximum effective target volume was formed within that specific time period. The revised target model could be also expanded to interpret the “two-hit” model of target theory, although the experimental data supported the “one-hit” model. If the “two-hit” model was sustained, theoretically, the target size would be 2.14 times larger than its original size to produce the same results.