The ovary of rice undergoes rapid expansion immediately after fertilization, and this process determines the final sink strength potential of caryopses. To date, progress on rice grain development mainly focuses on endosperm filling, whereas information on essential elements for ovary expansion remains limited. We report here the functional analysis of the ovary expansion retarded mutant crr1 in rice. Map-based cloning revealed that CRR1 encodes a protein homologous to Arabidopsis callose synthases AtGSL8 and AtGSL10. Point mutation in crr1 resulted in alternative splicing, which led to the formation of the truncated crr1 protein without the β-glucan synthase domain. Iodine staining showed that starch granules were few and unevenly distributed in the pericarp ofcrr1, and 5,6-CFDA transport assay revealed that carbohydrates were less efficiently unloaded from the lateral vasculatures into the developing caryopsis. CRR1 transcripts were detected in all plant organs, with the highest level found in receptacles, which are mainly composed of vascular tissues. Analysis of pCRR1::GUS transgenic plants showed that CRR1 was specifically expressed in vascular bundle cells. Consistently, loss of function of CRR1 led to disordered vascular cell patterns in the ovaries and receptacles of the mutant. Furthermore, a small portion of cells in the vascular bundles of crr1 showed defective cell wall formation, and callose deposition was specifically reduced at the plasmodesmata (PD) of cells with aberrant walls. Our results suggest that CRR1 performs a pivotal role in determining initial ovary expansion in rice possibly via the PD-mediated permeability of cell fate determinants for vascular cell differentiation.