The effects of potassium fertilizer (K2SO4) levels K0, (0), K1,(150) and K2,(250 kg/ha) in two species of Brassica napus (Hyola 401 Hybrid) and Brassica juncea (landrace cultivar), under three irrigation regimes, control (irrigation after 50%,), moderate stress, (irrigation after 70%), and severe stress (irrigation after 90% soil water depletion) were studied in a factorial experiment laid out in a randomized complete block design with three replications. Grain yield and physiological indices, including relative water content (RWC), stomatal conductance (g), chlorophyll content (SPAD values); leaf temperature (TL), and the difference between canopy temperature and air temperature (Tc-Ta) were measured at two stages (50% flowering and 100% siliques formation. Both species maintained, higher RWC, SPAD values and g, in non stress condition, but decreasing soil water supply caused a lower RWC, SPAD values, g, Dt and increased TL. Potassium application also improved above mentioned physiological traits. Grain yield was positively associated with RWC, g and SPAD values but showed a negative association with TL and Dt in both stages. Results showed that with increasing stress severity grain yield reduced significantly, but potassium application conferred great increase on rapeseed yield. Overall, grain yield showed significant association with RWC, g, SPAD values, Dt and TL under this experiment conditions. It is concluded that potassium application, could ameliorate negative effects of water stress on grain yield and physiological properties and consequently improved them. For selecting drought tolerant cultivars, due to easier measurement of g, SPAD values and TL, they could be recommended for screening large numbers of rapeseed cultivars in a short time at critical stages of crop growth.