Double-low rapeseed cultivars that produce no erucic acid and have only a small amount of glucosinolates are widely used. However, the roots in mature plant residue and leaves and roots of these seedlings still contain a large amount of biologically active glucosinolates, and it is important to clarify the effect of rapeseed cultivation on the subsequent crops. We analyzed the biological activities of the tissues of double-low cultivar at harvest. The laboratory bioassays revealed that the seeds and roots of mature rapeseed plants contained some autotoxic components that were volatile and water-soluble. We also analyzed the effects of root exudates from growing rapeseed plants on rapeseed germination and growth using the stair-step method and found that the root exudates were autotoxic. In the field, we investigated the allelopathic effects of rapeseed plants on the growth, yield, and quality of the subsequent sunflower plant in the same field. The growth, yield and thousand kernel weight of sunflower were reduced by the preceding cultivation of rapeseed. There were many spontaneously germinated juvenile rapeseed plants in the sunflower field preceded by rapeseed cultivation. Analysis of the plant and soil nitrogen (N) indicated that the growth reduction of sunflower was not caused by the competition for N with the spontaneously germinated juvenile rapeseed plants. The main factors responsible for poor sunflower growth under field after the cultivation of rapeseed were suggested to be some non-volatile, water-soluble components produced by the roots of the spontaneously germinated juvenile rapeseed.