In order to investigate the effect of different rates of phosphorous fertilizer on grain and oil yield and some agronomic traits of Indian mustard (Brassica juncea L.) under drought stress at early growth stage, a split plot experiment based on randomized complete block design was conducted with three replications at the Agricultural and Natural Resources Research Center of Sistan during 2009-2010 cropping season. Drought stress was assigned as the main plot factor with two levels including: 1- Non stress, where plots were irrigated from sowing to physiological maturity based on % 50 depletion of available water. 2- Vegetative phase drought stress, where plots were irrigated from sowing to flowering stage based on % 80 depletion of available water. Phosphorus fertilizer was considered as sub-plots with four levels of P₁=0, P₂=100, P₃=150 and P₄=200 kg.ha^-1 (from super-phosphate triple P2O5^-1). Results showed that early season drought stress (irrigation ceasing at rosette and budding to flowering stage) significantly decreased grain yield and yield components such as, number of siliques per plant, seeds per silique, 1000-grain weight, as well as plant height and oil yield traits. The greatest seed yield (3567 kg.ha^-1) and oil yield (1377 kg.ha^-1) were obtained at non- stress irrigation conditions. Increasing phosphorus application up to 150 kg.ha^-1 increased the grain yield and most related traits, while yield reduction was observed at 200 kg.ha^-1 level. The interaction effect of phosphorus application and irrigation on all traits was not significant. Amongst agronomic traits, the greatest correlation coefficients of grain yield were obtained with the number of silique on main branch (r= 0.84**), plant height (r=0.76**), silique number per plant (r= 0.74**), and 1000-seed weight (r=0.76**). Based on study results, to gain maximum grain and oil yield and yield components, water availability at vegetative growth stage (especially at budding stage) and at reproductive stage (flowering and grain filling stages) by consumption of 150 kg.ha^-1 super-phosphate triple is essential in mustard.