In angiosperms, seed development initiates from double fertilization, which produces a diploid embryo and a triploid endosperm. The endosperm, a terminally differentiated tissue that nourishes the embryo during seed development and germination, is the prominent tissue of imprinting in plants. Proper endosperm development is crucial for normal embryo and seed development. The expression of imprinted genes is regulated by epigenetic mechanisms, including DNA methylation, H3K27 trimethylation and PolIV-dependent siRNA (p4-siRNA). Such epigenetic regulation of imprinting is vital to proper endosperm development and seed viability. Recent studies show that endosperm DNA methylation is reduced genome-wide, which likely originates from demethylation in the central cell nucleus of the female gametophyte. This review focuses on the latest research advances in the epigenetic regulation in plant seed development, including the mechanism of plant genomic imprinting and dynamics of the genome-wide demethylation in endosperm.