?Mixed-planting of traditional and hybrid rice (Oryza sativa) varieties is an ecological approach for rice disease control and yield increase, in addition to its effective role in in situ conservation of traditional rice varieties. To estimate gene flow between traditional and hybrid rice varieties, an experiment involving Huangkenuo and Shanyou-63 was conducted to allow free gene flow by mixed-planting of the two varieties in different cultivation patterns.?A simple sequence repeat (SSR) marker RM167 was used to detect natural hybrids between the two varieties by examining seeds collected randomly from both varieties in the experiment.?The result showed an extremely low number of hybrids between the two varieties with an average frequency of 0.04% in Huangkenuo and 0.18% in Shanyou-63.?It is concluded that no significant gene flow occurs naturally between Huangkenuo and Shanyou-63, and that crop diversity management is a proper means for in situ conservation of traditional rice varieties. It is also implied that the potential transgene flow from transgenic hybrid rice to traditional rice variety would be extremely low.