The outcrossing rate of oilseed rape (Brassica napus) between two cultivars was assessed in an experimental field. During flowering (from the end April to the end of May), the wind from the pollen donor (♂) to the seed parent (♀) prevailed. The crossing rate (%) of the seed parent decreased as the distance from the pollen donor parent increased; 4.1% at 0.25 m distance, 1.4% at 1 m, 0.4% at 5 m, 0.15% at 10 m, 0.09% at 30 m, and 0.01% at 60 m. In another experiment, the hybridization rate between 55 cultivars of B. rapa (♀) and B. napus (♂) cultivars was assessed by placing rapa-planted pots in a napus field. The rate of 55 rapa cultivars ranged from 2% to 50% in the field, which might show the rapa cultivar-differences of crossability between the two species.