A field experiment was conducted in the new alluvial zone during rabi season of 2019-20 and 2020-21at District Seed Farm(22°93' N latitude, 88°53' E longitude and 9.75 m above mean sea level), Bidhan Chandra Krishi Viswavidyalaya, Kalyani, West Bengal, India to study the effect of spacing and fertilizer levels on yield of hybrid mustard (Brassica juncea M.). Kesari gold variety was planted under split plot design at two different spacings (S₁: 30x10 cm; S₂: 45x10 cm) as main plot treatment and three fertilizer levels (F₁: NPK@80-50-50 kg ha^-1; F₂: NPK@100-60-60 kg ha^-1; F₃: NPK@120-70-70 kg ha^-1) as subplot treatment. Among the spacings, the highest seed yield (1948 kg ha^-1 and 1887 kg ha^-1) was obtained with S₁. Highest seed yield (2018 kg ha^-1 and 1937 kg ha^-1) was recorded in the plots fertilized with F₃ in both years of study. Interaction of spacing and fertilizer had a non-significant effect on seed yield. Increase in the number of branches per plant, number of siliqua per plant, number of seeds per siliqua and harvest index was recorded with S₂ and F₃. Highest oil content was found in wider spacing crops but less fertilised plots. Benefit cost ratio obtained under interaction of different spacing and fertilizer levels revealed that the highest profit was earned from the S₁F₂ plot.