Abstract: Phytotoxic effects of aqueous extracts of walnut leaf were studied on germinating seeds and early seedling growth of mustard (Brassica campestris cv. Karanti) under western Himalayan agri-silvi system. Five treatments comprised of distilled water (control), 40, 60, 80 and 100% concentration of leaf extracts were treated. The effect of aqueous extracts was found inhibitive with concentration dependent manner on seed germination and subsequent seedling growth. The variety exhibited extent of phytotoxicity at 100% extracts application in comparison to untreated control. Invariably there was a decrease in first count, germination, seedling root and shoot length, seedling fresh and dry weight with increasing aqueous extracts concentration on germinating mustard. Present investigation shows that the tree species have allelopathic potential and contain water-soluble substances. They have inhibitory effects at higher concentrations but at low concentrations they have stimulatory effects. Seed germination, seedling elongation and weights were determined on date of final count, however, other seedling vigor i.e., vigor index, speed of germination index, Relative Growth Index (RGI), Mean Daily Germination (MDG), Mean Germination Time (MGT) and time to 50% germination (T50) were calculated as per their respective formula. The significant reduction in seed germination and seedling vigor were observed of walnut leaf extracts on mustard (Brassica campestris). It was found that seed germination and seedling vigor of mustard were affected negatively by walnut leaf extracts in concentration dependent manner.