Gene transferability from transgenic rapeseed to various subspecies and varieties of Brassica rapa was assessed in this study. Artificial crossability was studied in 118 cultivars of 7 B. rapa subspecies and varieties with the transgenic rapeseed GT73 (Brassica napus) as the pollen donor. On average 5.7 seeds were obtained per pollination, with a range from 0.05 to 19.4. The heading type of B. rapa L. showed significantly higher crossability than non-heading types of B. rapa. The spontaneous outcrossing rate between B. rapa (female) and the transgenic rapeseed Ms8 × Rf3 (B. napus) (male) ranged from 0.039 to 0.406%, with an average of 0.19%. The fertilization process and the development of the hybrid seeds as shown by fluorescent staining techniques indicated that the number of adhered pollens on the stigma was reduced by 80%, the number of pollen tubes in the style was reduced by 2/3 and the fertilization time was delayed by over 20 h when pollinated with the transgenic rapeseed Ms8 × Rf3 in comparison with the bud self-pollination of B. rapa as control. About 10–70% of the interspecific hybrid embryos were aborted in the course of development. Some seeds looked cracked in mature pods, which showed germination abilities lower than 10%. The spontaneous outcrossing rates were much lower than the artificial crossability, and their survival fitness of the interspecific hybrid was very low, indicating that it should be possible to keep the adventitious presence of the off-plants under the allowed threshold, if proper measures are taken.