Castor (Ricinus communis L.) is an oilseed crop, that is cultivated in arid and semi-arid region. Seed yield per plant of castor significantly effective by different environmental factors, but planting density also influence on seed yield. A field experiments were conducted in 2016 and 2017 to investigate the effects of in-row plant spacing (30, 40, 50 and 60 cm) on seed yield and weight in different portions of the raceme under drip irrigation along with mulching. The results indicating reduction in-row spacing, significantly reduced seed yield and weight per plant. Reduction percentage were greater in secondary and tertiary racemes as compared to primary racemes. Seed weight and yield in basal portion of raceme was also decreased by reducing in-row spacing. In comparison to raceme portion, seed weight was more reduced in the middle portion and showed no change in the upper portion of the raceme as compared to basal portion. Seed yield and weight per unit area was increased owing to the larger plant population with decreasing in-row spacing, but below the threshold value of in-row spacing (30 cm), seed fresh weight yield were decreased. Therefore, it was concluded, higher planting density by reducing the in-row spacing may increase castor fresh weight yield. Yield may be increased further by increasing the seed weight in the basal portion of the raceme, especially for primary racemes, while maintaining the seed weight in the upper and middle portions of the raceme. Further studies needed to explore different planting density effect on seed yield under different environmental factors within different portions of raceme.