Zhongshuang9, a new semi-winter Brassica napus variety with high resistance to Sclerotinia sclerotiorun and lodging, high-yield, double-low quality and extensive adaptability, was bred by multiple crossing and microspore culture technique. It was registered and released in China in 2002. In regional trial of Hubei Province in China, Zhongshuang9 yielded 2 482. 2 kg ha-1 averagely in 2000 - 2002, 15.33% higher than the control variety Zhongyou821. Erucic acid, glucosinolates and oil contents of Zhongshuang9 were 0.23%, 22.69 μmol g-1 (in meal) and 42%, respectively. In field assessment of resistance to S. Selerotiorum, the disease incidence and disease index of Zhongshuang9 averaged 13.31% and 6.47, respectively, which were lower than those of Zhongyou821 by 28% and 36%, respectively. After inoculation of detached leaves with mycelia, the lesion size of Zhongshuang9 was 4. 709 em2, which was significantly smaller than that of the mid-resistant variety Zhongyou821 (5. 933 em2 ). The stem lesion length of Zhongshuang9 after match-stick inoculation was 1. 275 cm, which was significantly lower than that of Zhongyou821 (1. 943 cm). The possible mechanism of resistance to S. sclerotiorum was studied through comparing the activities of phenylalanine ammonia lyase (PAL), exo-chitinase, β-1, 3-glucanase, peroxidase (POD) and polyphenoloxidase (PPO) in Zhongshuang9 with those in other resistant, mid-resistant and susceptible cultivars.