A field study was conducted at the experimental farm of Aligarh Muslim University, Aligarh on two cultivars of mustard (Brassia juncea L. Czern and Coss) namely “Alankar” and “PBM-16” under irrigated and non-irrigated conditions to assess the effect of ethrel (200 µLL-1) on the characteristics of growth and photosynthesis. Ethrel was sprayed at flowering stage [60 days after sowing (DAS)] to mustard crop. Leaf area index, plant dry mass, net photosynthetic rate (PN), internal CO2 concentration, stomatal conductance, transpiration rate, carboxylation efficiency, photosynthetic water use efficiency and plant water use efficiency were determined at 80 and 100 days after sowing (DAS). Leaf area index, plant dry mass, net photosynthetic rate, internal CO2 concentration, stomatal conductance, transpiration rate, carboxylation efficiency, photosynthetic water use efficiency and plant use efficiency were increased by 40.1, 25.0, 30.1, 15.8, 12.9, 9.9, 12.9, 20.8 and 15.1% respectively, by application of 200 µLL-1 ethrel.