Although genetically modified (GM) crops have never been cultivated commercially in Korea, it is necessary for a thorough assessment of the risks associated with their environmental release. We determined the frequency of pollen mediated gene flow from disease resistant GM rice (OsCK1) to non-GM rice (Nagdongbyeo) and weedy rice (R55). A total of 449,711 or 164,604 seeds were collected from non-GM and weedy rice, respectively which were planted around OsCK1. Resistance of the hybrids was determined by repeated spraying of herbicide and DNA analysis using specific primer to confirm hybrids. Though non-GM rice and weedy rice have similar flowering time, the hybrids were found only in non-GM rice and out-crossing ranged from 0.018% at 0.3 m to 0.013% at 0.6 m. All of hybrids were located within 0.6 m distance from the GM rice plot in southerly direction. The meteorological factors including temperature and relative humidity during flowering time were found to be the most important factors for determining rice out-crossing. It should be considered many factors like the local weather condition and flowering time to set up the safety management policy to prevent pollen mediated gene flow between GM and conventional crop.