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Progress through the cell cycle is a critical process during plant embryo and seed development and its progression is regulated by cyclins. Despite extensive study of cyclins in other systems, their role in embryo and seed development of maize is unclear. In this study, we demonstrate that <i>ZmCYCB1-1</i> overexpression significantly accelerated embryo growth and increased seed size. In situ hybridization and toluidine blue staining indicated that <i>ZmCYCB1-1</i> was highly expressed in the plumule of embryos, and the cells of the plumule were smaller, denser, and more regularly arranged in <i>ZmCYCB1-1</i> overexpression plants. Overexpression of <i>ZmCYCB1-1</i> in maize also resulted in an increased ear length and enhanced kernel weight by increasing kernel width. Transcriptome analysis indicated that the overexpression of <i>ZmCYCB1-1</i> affected several different metabolic pathways, including photosynthesis in embryos and leaves, and lipid metabolism in leaves. Conversely, knocking out <i>ZmCYCB1-1</i> resulted in plants with slow growth. Our results suggest that <i>ZmCYCB1-1</i> regulates embryo growth and seed size, making it an ideal target for efforts aimed at maize yield improvement.