Tubulin-folding cofactors play important roles in regulating plant development. Arabidopsis tubulin-folding cofactor B (AtTFC B) is an Arabidopsis homolog of mammalian tubulin-folding cofactor B, whose biological function in plant development remains poorly understood. Here we report that the homozygous attfc b (-/-) allele caused embryonic lethality. Embryogenesis was arrested at early embryo stage and the cells contained one or multiple nuclei. Plants carrying a heterozygous attfc b (+/-) allele exhibited enlarged mesophyll cells and leaf epidermal cells with bulged nuclei. Flow cytometry analysis showed increased ploidy in the leaves of the attfc b (+/-) mutant, as well as increased levels of Cdc2A and CycB1;1. In addition, immunofluorescence assay showed increased numbers of spindles and phragmoplasts in the attfc b (+/-) mutant. These results suggest that AtTFC B plays an important role in plant cell division.