Zinc (Zn) deficiency in crops, including rapeseed and mustard, is a widespread nutritional disorder especially in alkaline soils. However, plant analysis diagnostic criteria for interpreting Zn analysis in rapeseed and mustard are scarcely reported in the literature. Use of seeds for diagnosing Zn fertility status of soils has certain advantages over foliar analysis—ease of sampling, processing, and chemical analysis. Despite this, mature seeds of these species were hardly evaluated as an index tissue for this purpose. Our study determined Zn requirement in foliar tissues and also evaluated Zn composition of mature seeds as an index of Zn status of soils and plants. Zinc concentration in mature seeds of the test crops reflected the Zn status of the soil where plants were grown. In fact, the range of Zn concentration in grains was almost comparable with the ranges in foliar plant parts. Critical Zn concentration (mg/kg) in diagnostic plant parts of rapeseed was: whole shoots, 29; leaves, 33; and seeds, 29; while the Zn requirement of mustard was a little higher: whole shoots, 35; leaves, 41; and seeds, 33.