Abstract: This study investigated seed development in Sapium sebiferum var. conferticarpum Hu through a comprehensive analysis of external morphology, internal structure at different developmental stages, ultrastructure of the embryo and endosperm, and changes in nutritional components of the seed kernel. Results demonstrated that: (1) Fruit development in S. sebiferum var. conferticarpum was divided into three stages: fruit expansion stage (0–40 days post-flowering, DPF), kernel growth and development stage (40–100 DPF), and fruit maturation stage (100–160 DPF). Embryo formation was completed by 60 DPF, marking a critical stage in seed development. (2) Ultrastructural analysis revealed that oil bodies first emerged in both embryo and endosperm cells by 20 DPF. Between 60 and 100 DPF, oil body abundance increased rapidly, driving a 96.37% rise in total oil content. Notably, oil bodies within the endosperm matured earlier and remained smaller in diameter compared to those in the embryo. (3) Concurrently, soluble sugar, crude starch, and soluble protein levels exhibited an initial increase, followed by a decline and subsequent recovery, whereas oil content rose continuously throughout kernel development. Crude starch and soluble protein concentrations were inversely correlated with the activities of their respective enzymes (P<0.01). These results delineate the temporal dynamics of oil deposition during seed development in S. sebiferum var. conferticarpum and offer a mechanistic framework to inform precision cultivation practices and the genetic improvement of high-oil-yielding germplasms.